#38 iteration 1: file attachments, 1 file per paste

- internal/store/blob.go: BlobStore interface + fs implementation with
  traversal-safe keys (<paste-id>/<sha256>), put/get/stat/delete
- attachments table migration (id, paste_id, filename sanitized to 255,
  mime sniffed server-side, size, sha256, created_at)
- POST /api/pastes now accepts multipart/form-data with a 'file' part;
  1 file = 1 paste: file replaces text content when both are sent
- 25 MB per-file limit enforced server-side (413 file_too_large)
- GET /f/{attachment-id}/{filename}: stored sniffed mime, nosniff,
  inline only for images/pdf, html/svg/xml forced to text/plain (#34 rule)
- paste view renders attachment chip + inline image preview
- /new: dropzone with file picker, drag-and-drop, Ctrl+V file paste,
  file chip with name/size/remove, matches pill/radius design
- tests: blob roundtrip/traversal/sanitize; multipart create (mime
  sniffing, client mime ignored, size limit, two-file reject, html/svg
  forcing, 404s, password/expiry fields)

.gitignore

@@ -4,3 +4,4 @@ palette.db-shm
 palette.db-wal
 admin-key
 settings.json
+:memory:.files/

README.md

@@ -63,6 +63,7 @@ go build -o palette ./cmd/palette
 | `PALETTE_MAX_TEXT` | `5242880` | Max paste size in bytes (5 MB) |
 | `PALETTE_MAX_ITEM` | `26214400` | Max can item size in bytes (25 MB) |
 | `PALETTE_ADMIN_KEY` | generated | Admin key; if unset a 32-char hex key is generated and persisted to `<db-dir>/admin-key` (0600) |
+| `PALETTE_DEFAULT_DARK` | dark on | Default dark mode for new visitors. Set `false`, `0`, or `off` to default to light mode. Visitors who toggle dark mode keep their choice in their browser. |
 
 An `/admin` page exists for runtime settings, protected by a key set at
 install (`PALETTE_ADMIN_KEY` env var) and resettable locally — see

cmd/palette/main.go

@@ -42,6 +42,7 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
+	web.SetDefaultDark(os.Getenv("PALETTE_DEFAULT_DARK"))
 	srv := api.NewServer(st, cfg, ui, ss, adminKey)
 	log.Printf("palette listening on %s", cfg.Addr)
 	log.Fatal(http.ListenAndServe(cfg.Addr, srv.Routes()))

internal/api/attachments.go

@@ -0,0 +1,351 @@
+package api
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+
+	"palette/internal/store"
+)
+
+// #38: file attachments, iteration 1: one file per paste. A paste either has
+// text content OR one attached file. Multipart create + /f/ serving route.
+
+const (
+	MaxAttachmentBytes = 25 << 20                   // 25 MB per file
+	maxFileBytesHard   = MaxAttachmentBytes + 1<<20 // sniff headroom; over this reject before reading it all
+)
+
+// sniffMime runs http.DetectContentType on the first 512 bytes (and any
+// remainder of the head) of r, returning the sniffed mime and a reader that
+// replays the full stream. Mime is NEVER taken from the client.
+func sniffMime(r io.Reader) (string, io.Reader, error) {
+	head := make([]byte, 512)
+	n, err := io.ReadFull(r, head)
+	if err != nil && err != io.ErrUnexpectedEOF && err != io.EOF {
+		return "", nil, err
+	}
+	head = head[:n]
+	mime := http.DetectContentType(head)
+	return mime, io.MultiReader(bytes.NewReader(head), r), nil
+}
+
+// sanitizeMimeForServing maps the stored (sniffed) mime to the Content-Type
+// used on /f/. Active-content types (html, svg, xml...) are forced to
+// text/plain — same rule as the /raw #34 fix — so a malicious upload can
+// never execute on this origin.
+func serveContentType(mime string) string {
+	base := mime
+	if i := strings.IndexByte(mime, ';'); i >= 0 {
+		base = strings.TrimSpace(mime[:i])
+	}
+	base = strings.ToLower(base)
+	switch base {
+	case "text/html", "image/svg+xml", "application/xhtml+xml", "text/xml",
+		"application/xml", "application/xhtml", "image/xml+svg":
+		return "text/plain; charset=utf-8"
+	}
+	return mime
+}
+
+// inlineable reports whether the sniffed mime is safe to render inline
+// (Content-Disposition: inline); everything else downloads as an attachment.
+func inlineable(mime string) bool {
+	base := mime
+	if i := strings.IndexByte(mime, ';'); i >= 0 {
+		base = strings.TrimSpace(mime[:i])
+	}
+	base = strings.ToLower(base)
+	switch {
+	case strings.HasPrefix(base, "image/"), base == "application/pdf":
+		return true
+	}
+	return false
+}
+
+// mime/multipart parts are fully read during parsing (the multipart reader
+// closes each part when advancing) and the mime is sniffed from bytes.
+
+// limitAttachment rejects reads past the 25 MB per-file cap server-side.
+type limitReader struct {
+	r   io.Reader
+	n   int64
+	max int64
+}
+
+func (l *limitReader) Read(p []byte) (int, error) {
+	if l.n > l.max {
+		return 0, store.ErrFileTooLarge
+	}
+	n, err := l.r.Read(p)
+	l.n += int64(n)
+	if l.n > l.max && err == nil {
+		err = store.ErrFileTooLarge
+	}
+	return n, err
+}
+
+// handleCreatePasteMultipart implements POST /api/pastes with
+// multipart/form-data (#38). Fields mirror the JSON create path; a 'file'
+// part makes the paste a file paste (1 file = 1 paste: if text content is
+// also present, the file wins and the text is ignored — simplest correct
+// behavior, documented in the PR).
+func (a *apiServer) handleCreatePasteMultipart(w http.ResponseWriter, r *http.Request, s Settings) {
+	blobs := a.store.Blobs()
+	if blobs == nil {
+		writeErr(w, 500, "blob storage unavailable")
+		return
+	}
+	// guard the raw body: 25 MB file + multipart overhead headroom
+	mr, err := r.MultipartReader()
+	if err != nil {
+		writeErr(w, 400, "invalid multipart body")
+		return
+	}
+	var (
+		p        store.Paste
+		fileSeen bool
+		att      store.Attachment
+		fileBody io.Reader
+	)
+	for {
+		part, err := mr.NextPart()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			if isBodyTooLarge(err) {
+				writeErrCode(w, http.StatusRequestEntityTooLarge, "content_too_large", "request body too large")
+				return
+			}
+			writeErr(w, 400, "invalid multipart body")
+			return
+		}
+		name := part.FormName()
+		if name == "file" {
+			if fileSeen {
+				writeErrCode(w, 400, "one_file_only", "Choose either text or a file for now. Only one file per paste.")
+				part.Close()
+				return
+			}
+			// The part must be fully read during parsing: the multipart
+			// reader closes it as soon as the next part is fetched. Read it
+			// here into memory (bounded by the 25 MB cap) and sniff the mime
+			// from the content, never from client headers.
+			limited := io.LimitReader(part, MaxAttachmentBytes+1)
+			raw, err := io.ReadAll(limited)
+			part.Close()
+			if err != nil {
+				writeErr(w, 400, "invalid file part")
+				return
+			}
+			if int64(len(raw)) > MaxAttachmentBytes {
+				writeErrCode(w, http.StatusRequestEntityTooLarge, "file_too_large",
+					"File is too large. The limit is 25 MB.")
+				return
+			}
+			if len(raw) == 0 {
+				writeErrCode(w, 400, "content_empty", "The file is empty.")
+				return
+			}
+			filename := store.SanitizeFilename(part.FileName())
+			mime := http.DetectContentType(raw[:min(512, len(raw))])
+			att = store.Attachment{PasteID: "pending", Filename: filename, Mime: mime}
+			fileBody = bytes.NewReader(raw)
+			fileSeen = true
+			continue
+		}
+		val, err := io.ReadAll(io.LimitReader(part, 1<<16))
+		part.Close()
+		if err != nil {
+			writeErr(w, 400, "invalid multipart field")
+			return
+		}
+		v := string(val)
+		switch name {
+		case "content":
+			p.Content = v
+		case "title":
+			p.Title = &v
+		case "language":
+			p.Language = &v
+		case "custom_slug":
+			p.CustomSlug = &v
+		case "password":
+			p.Password = &v
+		case "expires_in":
+			p.ExpiresIn = &v
+		case "visibility":
+			p.Visibility = v
+		case "burn_after_read":
+			p.BurnAfterRead = v == "true" || v == "1" || v == "on"
+		case "burn_after_reads":
+			if n, err := strconv.Atoi(v); err == nil {
+				p.BurnAfterReads = &n
+			}
+		}
+	}
+
+	if fileSeen {
+		// 1 file = 1 paste: the file replaces text content.
+		p.Content = ""
+	} else if status, msg := checkContent(p.Content, s.MaxContentBytes); status != 0 {
+		if status == http.StatusRequestEntityTooLarge {
+			writeErrCode(w, status, "content_too_large", msg)
+		} else {
+			writeErrCode(w, status, "content_empty", msg)
+		}
+		return
+	}
+	// #86 metadata bounds + default expiry: same rules as the JSON path
+	if p.Title != nil {
+		t, err := checkTitle(*p.Title)
+		if err != nil {
+			writeErr(w, 400, err.Error())
+			return
+		}
+		p.Title = &t
+	}
+	if p.Language != nil {
+		l, err := checkLanguage(*p.Language)
+		if err != nil {
+			writeErr(w, 400, err.Error())
+			return
+		}
+		if l == "" {
+			p.Language = nil
+		} else {
+			p.Language = &l
+		}
+	}
+	if p.BurnAfterReads != nil {
+		if err := parseBurnAfterReads(*p.BurnAfterReads); err != nil {
+			writeErr(w, 400, err.Error())
+			return
+		}
+	}
+	if (p.ExpiresIn == nil || *p.ExpiresIn == "") && s.DefaultExpiry != "" {
+		def := s.DefaultExpiry
+		p.ExpiresIn = &def
+	}
+	p.ViewerID = currentViewerID(r)
+
+	created, err := a.store.CreatePaste(&p)
+	if err != nil {
+		writeErrCode(w, 400, createErrCode(err), err.Error())
+		return
+	}
+
+	resp := map[string]any{
+		"id":             created.ID,
+		"deletion_token": created.DeletionToken,
+		"url":            "/" + created.ID,
+		"raw_url":        "/raw/" + created.ID,
+		"api_url":        "/api/pastes/" + created.ID,
+		"expires_at":     created.ExpiresAt,
+		"created_at":     created.CreatedAt,
+		"rate_limit":     map[string]int{"create_per_sec": 1, "burst": 5},
+	}
+
+	if fileSeen {
+		att.PasteID = created.ID
+		// size pre-check happens inside the limited read; re-run with limit
+		// enforced so oversized uploads fail before the blob is stored.
+		err := a.store.CreateAttachment(&att, fileBody, blobs)
+		if err == store.ErrFileTooLarge {
+			a.store.SoftDelete(created.ID)
+			writeErrCode(w, http.StatusRequestEntityTooLarge, "file_too_large",
+				"File is too large. The limit is 25 MB.")
+			return
+		}
+		if err != nil {
+			a.store.SoftDelete(created.ID)
+			writeErr(w, 500, "could not store file")
+			return
+		}
+		resp["attachment"] = map[string]any{
+			"id":       att.ID,
+			"filename": att.Filename,
+			"mime":     att.Mime,
+			"size":     att.Size,
+			"sha256":   att.SHA256,
+			"url":      "/f/" + att.ID + "/" + att.Filename,
+		}
+	}
+	writeJSON(w, 201, resp)
+}
+
+// handleServeAttachment serves GET /f/{attachment-id}/{filename} with the
+// stored (server-sniffed) mime, nosniff, and a safe Content-Disposition.
+// The filename path segment is decorative; lookups key on the attachment id.
+func (a *apiServer) handleServeAttachment(w http.ResponseWriter, r *http.Request) {
+	id := chi.URLParam(r, "aid")
+	att, err := a.store.GetAttachment(id)
+	if err != nil {
+		writeErr(w, 500, "db error")
+		return
+	}
+	if att == nil {
+		writeErr(w, 404, "attachment not found")
+		return
+	}
+	// attachment inherits the paste's lifecycle: gone if the paste is gone
+	row, err := a.store.GetPaste(att.PasteID)
+	if err != nil || row == nil {
+		writeErr(w, 404, "attachment not found")
+		return
+	}
+	if row.ExpiresAt.Valid && row.ExpiresAt.Int64 < time.Now().Unix() {
+		writeErr(w, 404, "attachment not found")
+		return
+	}
+	if row.Burned() {
+		writeErr(w, 404, "attachment not found")
+		return
+	}
+
+	blobs := a.store.Blobs()
+	if blobs == nil {
+		writeErr(w, 500, "blob storage unavailable")
+		return
+	}
+	blob, err := blobs.Get(att.PasteID + "/" + att.SHA256)
+	if err != nil {
+		writeErr(w, 404, "attachment not found")
+		return
+	}
+	defer blob.Close()
+
+	ct := serveContentType(att.Mime)
+	w.Header().Set("Content-Type", ct)
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+	disposition := "attachment"
+	if inlineable(att.Mime) {
+		disposition = "inline"
+	}
+	w.Header().Set("Content-Disposition",
+		fmt.Sprintf(`%s; filename="%s"`, disposition, asciiFilename(att.Filename)))
+	w.Header().Set("Content-Length", fmt.Sprintf("%d", att.Size))
+	http.ServeContent(w, r, "", time.Unix(att.CreatedAt, 0), blob)
+}
+
+// asciiFilename quotes a filename for the Content-Disposition header,
+// escaping quotes and backslashes and dropping non-ASCII bytes.
+func asciiFilename(name string) string {
+	var b strings.Builder
+	for _, r := range name {
+		if r < 128 && r != '"' && r != '\\' && r > 31 {
+			b.WriteRune(r)
+		}
+	}
+	if b.Len() == 0 {
+		return "file"
+	}
+	return b.String()
+}

internal/api/attachments_test.go

@@ -0,0 +1,308 @@
+package api
+
+import (
+	"bytes"
+	"encoding/json"
+	"mime/multipart"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"palette/internal/store"
+)
+
+// multipartCreate posts a multipart create to the routes handler; extra
+// fields are appended as text parts. Returns recorder and parsed response.
+func multipartCreate(t *testing.T, h http.Handler, filename string, content []byte, fields map[string]string) (*httptest.ResponseRecorder, map[string]any) {
+	t.Helper()
+	var buf bytes.Buffer
+	mw := multipart.NewWriter(&buf)
+	if filename != "" {
+		fw, _ := mw.CreateFormFile("file", filename)
+		fw.Write(content)
+	}
+	for k, v := range fields {
+		mw.WriteField(k, v)
+	}
+	mw.Close()
+	req := httptest.NewRequest("POST", "/api/pastes", &buf)
+	req.Header.Set("Content-Type", mw.FormDataContentType())
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	var resp map[string]any
+	json.Unmarshal(rec.Body.Bytes(), &resp)
+	return rec, resp
+}
+
+func TestMultipartAttachmentCreateAndServe(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+
+	png := append([]byte("\x89PNG\r\n\x1a\n"), bytes.Repeat([]byte{0, 1, 2, 3}, 32)...)
+	rec, resp := multipartCreate(t, h, "shot.png", png, map[string]string{"title": "with file"})
+	if rec.Code != 201 {
+		t.Fatalf("create: %d %s", rec.Code, rec.Body.String())
+	}
+	att, _ := resp["attachment"].(map[string]any)
+	if att == nil {
+		t.Fatalf("no attachment in response: %v", resp)
+	}
+	id, _ := att["id"].(string)
+	url, _ := att["url"].(string)
+	if url != "/f/"+id+"/shot.png" {
+		t.Fatalf("attachment url = %q", url)
+	}
+	if att["mime"] != "image/png" {
+		t.Fatalf("sniffed mime = %v want image/png", att["mime"])
+	}
+
+	// serve: image mime -> inline, nosniff, stored bytes
+	req := httptest.NewRequest("GET", url, nil)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req)
+	if rec2.Code != 200 {
+		t.Fatalf("serve: %d %s", rec2.Code, rec2.Body.String())
+	}
+	if got := rec2.Header().Get("Content-Type"); got != "image/png" {
+		t.Fatalf("Content-Type = %q", got)
+	}
+	if got := rec2.Header().Get("X-Content-Type-Options"); got != "nosniff" {
+		t.Fatalf("nosniff = %q", got)
+	}
+	if got := rec2.Header().Get("Content-Disposition"); !strings.HasPrefix(got, "inline") {
+		t.Fatalf("Content-Disposition = %q", got)
+	}
+	if !bytes.Equal(rec2.Body.Bytes(), png) {
+		t.Fatal("served bytes differ from upload")
+	}
+}
+
+func TestMultipartFileReplacesText(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	rec, resp := multipartCreate(t, h, "notes.txt", []byte("file body"), map[string]string{"content": "some text"})
+	if rec.Code != 201 {
+		t.Fatalf("create: %d %s", rec.Code, rec.Body.String())
+	}
+	id, _ := resp["id"].(string)
+	req := httptest.NewRequest("GET", "/api/pastes/"+id, nil)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req)
+	var got map[string]any
+	json.Unmarshal(rec2.Body.Bytes(), &got)
+	if got["content"] != "" {
+		t.Fatalf("content should be empty when file provided, got %v", got["content"])
+	}
+}
+
+func TestMultipartSecondFileRejected(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	var buf bytes.Buffer
+	mw := multipart.NewWriter(&buf)
+	for _, name := range []string{"a.txt", "b.txt"} {
+		fw, _ := mw.CreateFormFile("file", name)
+		fw.Write([]byte("x"))
+	}
+	mw.Close()
+	req := httptest.NewRequest("POST", "/api/pastes", &buf)
+	req.Header.Set("Content-Type", mw.FormDataContentType())
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 400 {
+		t.Fatalf("two files: got %d want 400", rec.Code)
+	}
+	if !strings.Contains(rec.Body.String(), "one_file_only") {
+		t.Fatalf("error code missing: %s", rec.Body.String())
+	}
+}
+
+func TestMultipartHtmlUploadServesAsPlainText(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	html := []byte("<html><script>alert(1)</script></html>")
+	rec, resp := multipartCreate(t, h, "page.html", html, nil)
+	if rec.Code != 201 {
+		t.Fatalf("create: %d %s", rec.Code, rec.Body.String())
+	}
+	url, _ := resp["attachment"].(map[string]any)["url"].(string)
+	req := httptest.NewRequest("GET", url, nil)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req)
+	if got := rec2.Header().Get("Content-Type"); got != "text/plain; charset=utf-8" {
+		t.Fatalf("html served as %q, want text/plain", got)
+	}
+	if got := rec2.Header().Get("Content-Disposition"); !strings.HasPrefix(got, "attachment") {
+		t.Fatalf("html Content-Disposition = %q, want attachment", got)
+	}
+}
+
+func TestMultipartSvgUploadServesAsPlainText(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	svg := []byte(`<?xml version="1.0"?><svg xmlns="http://www.w3.org/2000/svg"><script>alert(1)</script></svg>`)
+	rec, resp := multipartCreate(t, h, "evil.svg", svg, nil)
+	if rec.Code != 201 {
+		t.Fatalf("create: %d %s", rec.Code, rec.Body.String())
+	}
+	url, _ := resp["attachment"].(map[string]any)["url"].(string)
+	req := httptest.NewRequest("GET", url, nil)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req)
+	ct := rec2.Header().Get("Content-Type")
+	if strings.Contains(ct, "svg") || strings.Contains(ct, "html") {
+		t.Fatalf("svg served as %q", ct)
+	}
+	if ct != "text/plain; charset=utf-8" {
+		t.Fatalf("svg Content-Type = %q", ct)
+	}
+}
+
+func TestMultipartClientMimeIgnored(t *testing.T) {
+	// client claims image/png; server must sniff the real type (text)
+	s := testServer(t)
+	h := s.routes()
+	var buf bytes.Buffer
+	mw := multipart.NewWriter(&buf)
+	fw, _ := mw.CreateFormFile("file", "fake.png")
+	fw.Write([]byte("just plain text, definitely not a png"))
+	// note: CreateFormFile sets Content-Type: application/octet-stream; the
+	// sniffed type for text content is text/plain either way.
+	mw.Close()
+	req := httptest.NewRequest("POST", "/api/pastes", &buf)
+	req.Header.Set("Content-Type", mw.FormDataContentType())
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 201 {
+		t.Fatalf("create: %d %s", rec.Code, rec.Body.String())
+	}
+	var resp map[string]any
+	json.Unmarshal(rec.Body.Bytes(), &resp)
+	att := resp["attachment"].(map[string]any)
+	if att["mime"] != "text/plain; charset=utf-8" && att["mime"] != "text/plain" {
+		t.Fatalf("mime = %v, want sniffed text/plain", att["mime"])
+	}
+}
+
+func TestMultipartOversizeRejected(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	big := bytes.Repeat([]byte("A"), MaxAttachmentBytes+1024)
+	rec, _ := multipartCreate(t, h, "big.bin", big, nil)
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("oversize: got %d want 413", rec.Code)
+	}
+}
+
+func TestMultipartExactlyAtLimitAccepted(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	exact := bytes.Repeat([]byte("A"), MaxAttachmentBytes)
+	rec, resp := multipartCreate(t, h, "exact.bin", exact, nil)
+	if rec.Code != 201 {
+		t.Fatalf("at-limit: got %d %s", rec.Code, rec.Body.String())
+	}
+	att := resp["attachment"].(map[string]any)
+	if att["size"].(float64) != float64(MaxAttachmentBytes) {
+		t.Fatalf("size = %v", att["size"])
+	}
+}
+
+func TestMultipartEmptyFileRejected(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	rec, _ := multipartCreate(t, h, "empty.txt", nil, nil)
+	if rec.Code != 400 {
+		t.Fatalf("empty file: got %d want 400", rec.Code)
+	}
+}
+
+func TestServeAttachment404Missing(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	req := httptest.NewRequest("GET", "/f/zzzzzzzz/nonexistent.txt", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 404 {
+		t.Fatalf("missing attachment: got %d want 404", rec.Code)
+	}
+}
+
+func TestServeAttachmentUnknownPaste404(t *testing.T) {
+	// attachment row referencing a paste that doesn't exist must 404, not leak
+	s := testServer(t)
+	h := s.routes()
+	s.store.CreatePaste(&store.Paste{Content: "x"})
+	att := store.Attachment{PasteID: "ghost00", Filename: "f.txt", Mime: "text/plain"}
+	blobs := s.store.Blobs()
+	if err := s.store.CreateAttachment(&att, strings.NewReader("hello"), blobs); err != nil {
+		t.Fatal(err)
+	}
+	req := httptest.NewRequest("GET", "/f/"+att.ID+"/f.txt", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != 404 {
+		t.Fatalf("orphan attachment: got %d want 404", rec.Code)
+	}
+}
+
+func TestServeAttachmentPdfInline(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	pdf := []byte("%PDF-1.4\n%fake pdf body\n")
+	rec, resp := multipartCreate(t, h, "doc.pdf", pdf, nil)
+	if rec.Code != 201 {
+		t.Fatalf("create: %d %s", rec.Code, rec.Body.String())
+	}
+	url, _ := resp["attachment"].(map[string]any)["url"].(string)
+	req := httptest.NewRequest("GET", url, nil)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req)
+	if got := rec2.Header().Get("Content-Type"); !strings.HasPrefix(got, "application/pdf") {
+		t.Fatalf("pdf Content-Type = %q", got)
+	}
+	if got := rec2.Header().Get("Content-Disposition"); !strings.HasPrefix(got, "inline") {
+		t.Fatalf("pdf Content-Disposition = %q", got)
+	}
+}
+
+func TestServeAttachmentBurnedPaste404(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	rec, resp := multipartCreate(t, h, "burn.txt", []byte("burn me"), nil)
+	if rec.Code != 201 {
+		t.Fatalf("create: %d", rec.Code)
+	}
+	att := resp["attachment"].(map[string]any)
+	url, _ := att["url"].(string)
+	pid, _ := resp["id"].(string)
+	// burn the paste via API read (burn_after_read default off here, so force)
+	s.store.SoftDelete(pid)
+	req := httptest.NewRequest("GET", url, nil)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req)
+	if rec2.Code != 404 {
+		t.Fatalf("deleted paste attachment: got %d want 404", rec2.Code)
+	}
+}
+
+func TestMultipartPasswordFieldAccepted(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	rec, resp := multipartCreate(t, h, "secret.txt", []byte("top secret"),
+		map[string]string{"password": "hunter2", "expires_in": "1h"})
+	if rec.Code != 201 {
+		t.Fatalf("create: %d %s", rec.Code, rec.Body.String())
+	}
+	if resp["attachment"] == nil {
+		t.Fatal("attachment missing")
+	}
+	id, _ := resp["id"].(string)
+	req := httptest.NewRequest("GET", "/api/pastes/"+id, nil)
+	rec2 := httptest.NewRecorder()
+	h.ServeHTTP(rec2, req)
+	if rec2.Code != 401 {
+		t.Fatalf("paste should require password, got %d", rec2.Code)
+	}
+}

internal/api/server.go

@@ -113,7 +113,8 @@ func (a *apiServer) routes() http.Handler {
 	r.Get("/can/{id}", a.handleCanPage)
 	r.Post("/can/{id}", a.handleCanPage)
 
-	// raw
+	// raw + #38 attachment serving
+	r.Get("/f/{aid}/{filename}", a.handleServeAttachment)
 	r.Get("/raw/{id}", a.handleRaw)
 
 	// web pages
@@ -180,6 +181,11 @@ func (a *apiServer) handleCreatePaste(w http.ResponseWriter, r *http.Request) {
 		writeRateLimited(w, 1)
 		return
 	}
+	ct := r.Header.Get("Content-Type")
+	if strings.HasPrefix(ct, "multipart/form-data") {
+		a.handleCreatePasteMultipart(w, r, s) // #38: file attachment create
+		return
+	}
 	var p store.Paste
 	if err := json.NewDecoder(r.Body).Decode(&p); err != nil {
 		if isBodyTooLarge(err) { // #68: body cut off by MaxBytesReader

internal/api/validate.go

@@ -18,6 +18,10 @@ import (
 func (a *apiServer) maxRequestBody() int64 {
 	s := a.settings.get()
 	max := s.MaxContentBytes + 4096
+	// #38: multipart create carries up to one 25 MB file plus overhead
+	if a.cfg.MaxItemBytes+1<<20 > max {
+		max = a.cfg.MaxItemBytes + 1<<20
+	}
 	if max < 64*1024 {
 		max = 64 * 1024
 	}

internal/store/attachment.go

@@ -0,0 +1,147 @@
+package store
+
+import (
+	"crypto/sha256"
+	"database/sql"
+	"errors"
+	"fmt"
+	"path/filepath"
+	"strings"
+	"time"
+)
+
+// #38: file attachments, one file per paste (iteration 1). A paste either
+// has text content OR one attached file. Bytes live in the BlobStore; this
+// table carries the metadata.
+
+type Attachment struct {
+	ID        string `json:"id"`
+	PasteID   string `json:"paste_id"`
+	Filename  string `json:"filename"`
+	Mime      string `json:"mime"`
+	Size      int64  `json:"size"`
+	SHA256    string `json:"sha256"`
+	CreatedAt int64  `json:"created_at"`
+
+	SizeHuman string `json:"-"` // template-only: human-readable size
+}
+
+const MaxFilenameLen = 255
+
+// ErrFileTooLarge is returned when an attachment exceeds the per-file cap.
+var ErrFileTooLarge = errors.New("file too large")
+
+// SanitizeFilename cleans a user-supplied filename: strips directory
+// components, control chars, and caps at 255 bytes. The extension is never
+// trusted for mime decisions (mime is sniffed server-side).
+func SanitizeFilename(name string) string {
+	name = filepath.Base(strings.ReplaceAll(name, "\\", "/"))
+	name = strings.TrimSpace(name)
+	var b strings.Builder
+	for _, r := range name {
+		if r < 32 || r == 127 {
+			continue
+		}
+		b.WriteRune(r)
+	}
+	name = b.String()
+	if len(name) > MaxFilenameLen {
+		runes := []rune(name)
+		for len(string(runes)) > MaxFilenameLen {
+			runes = runes[:len(runes)-1]
+		}
+		name = string(runes)
+	}
+	if name == "" || name == "." || name == ".." {
+		name = "file"
+	}
+	return name
+}
+
+// CreateAttachment stores the file bytes and inserts the attachments row.
+// The mime MUST already be sniffed server-side (http.DetectContentType by
+// the caller); it is never taken from the client.
+func (s *Store) CreateAttachment(a *Attachment, r interface{ Read([]byte) (int, error) }, blobs BlobStore) error {
+	pasteID := a.PasteID
+	sha, size, err := blobs.Put(pasteID+"/pending", r)
+	if err != nil {
+		return err
+	}
+	if a.Size > 0 && size > a.Size {
+		// caller-provided pre-check limit; treat as too large
+		blobs.Delete(pasteID + "/" + sha)
+		return ErrFileTooLarge
+	}
+	a.SHA256 = sha
+	a.Size = size
+	now := time.Now().Unix()
+	id := genSlug(20)
+	_, err = s.db.Exec(`INSERT INTO attachments (id, paste_id, filename, mime, size, sha256, created_at)
+		VALUES (?,?,?,?,?,?,?)`, id, pasteID, a.Filename, a.Mime, size, sha, now)
+	if err != nil {
+		blobs.Delete(pasteID + "/" + sha)
+		return err
+	}
+	a.ID = id
+	a.CreatedAt = now
+	return nil
+}
+
+// GetAttachment returns the attachment row for a paste, or nil.
+// One file per paste (iteration 1), so a single-row lookup keyed on paste.
+func (s *Store) GetAttachmentForPaste(pasteID string) (*Attachment, error) {
+	row := s.db.QueryRow(`SELECT id, paste_id, filename, mime, size, sha256, created_at
+		FROM attachments WHERE paste_id = ? ORDER BY created_at ASC LIMIT 1`, pasteID)
+	return scanAttachment(row)
+}
+
+func (s *Store) GetAttachment(id string) (*Attachment, error) {
+	row := s.db.QueryRow(`SELECT id, paste_id, filename, mime, size, sha256, created_at
+		FROM attachments WHERE id = ?`, id)
+	return scanAttachment(row)
+}
+
+func scanAttachment(row *sql.Row) (*Attachment, error) {
+	var a Attachment
+	err := row.Scan(&a.ID, &a.PasteID, &a.Filename, &a.Mime, &a.Size, &a.SHA256, &a.CreatedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	a.SizeHuman = humanBytes(a.Size)
+	return &a, nil
+}
+
+// humanBytes formats a byte count for display (KB/MB).
+func humanBytes(n int64) string {
+	switch {
+	case n >= 1<<20:
+		return fmt.Sprintf("%.1f MB", float64(n)/(1<<20))
+	case n >= 1<<10:
+		return fmt.Sprintf("%.1f KB", float64(n)/(1<<10))
+	default:
+		return fmt.Sprintf("%d B", n)
+	}
+}
+
+// DeleteAttachment removes the row and its blob.
+func (s *Store) DeleteAttachment(a *Attachment, blobs BlobStore) error {
+	blobs.Delete(a.PasteID + "/" + a.SHA256)
+	_, err := s.db.Exec(`DELETE FROM attachments WHERE id = ?`, a.ID)
+	return err
+}
+
+// HasAttachment reports whether a paste already carries a file (#38: one file per paste).
+func (s *Store) HasAttachment(pasteID string) bool {
+	var n int
+	s.db.QueryRow(`SELECT COUNT(*) FROM attachments WHERE paste_id = ?`, pasteID).Scan(&n)
+	return n > 0
+}
+
+// HashBytes is a small helper used by handlers to name/verify blobs.
+func HashBytes(b []byte) string {
+	h := sha256.Sum256(b)
+	return string(h[:])
+}

internal/store/blob.go

@@ -0,0 +1,139 @@
+package store
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// #38: blob storage abstraction. Files live outside SQLite on the data
+// volume behind this interface so a MinIO/S3 backend can replace the fs
+// implementation later without touching handlers or the UI.
+
+// ErrBlobNotFound is returned by Get/Stat when the key does not exist.
+var ErrBlobNotFound = errors.New("blob not found")
+
+// BlobStore persists attachment bytes by key.
+type BlobStore interface {
+	Put(key string, r io.Reader) (sha string, size int64, err error)
+	Get(key string) (io.ReadSeekCloser, error)
+	Delete(key string) error
+	Stat(key string) (int64, error)
+}
+
+// FsBlobStore is the filesystem implementation: blobs are stored under
+// root/<key>, where key is "<paste-id>/<sha256-hex>" (see SanitizeBlobKey).
+type FsBlobStore struct {
+	root string
+}
+
+// NewFsBlobStore creates the blob root directory.
+func NewFsBlobStore(root string) (*FsBlobStore, error) {
+	if err := os.MkdirAll(root, 0o700); err != nil {
+		return nil, err
+	}
+	return &FsBlobStore{root: root}, nil
+}
+
+// SanitizeBlobKey validates a blob key and returns the safe on-disk path
+// under root. Keys must be exactly "<paste-id>/<sha256-hex>"; anything with
+// separators outside that shape, "..", or absolute paths is rejected, so a
+// crafted key can never escape the blob root (traversal).
+func (f *FsBlobStore) path(key string) (string, error) {
+	clean := filepath.ToSlash(key)
+	parts := strings.Split(clean, "/")
+	if len(parts) != 2 || parts[0] == "" || parts[1] == "" ||
+		parts[0] == "." || parts[0] == ".." || parts[1] == "." || parts[1] == ".." {
+		return "", fmt.Errorf("invalid blob key %q", key)
+	}
+	return filepath.Join(f.root, filepath.FromSlash(clean)), nil
+}
+
+func (f *FsBlobStore) Put(key string, r io.Reader) (string, int64, error) {
+	dst, err := f.path(key)
+	if err != nil {
+		return "", 0, err
+	}
+	if err := os.MkdirAll(filepath.Dir(dst), 0o700); err != nil {
+		return "", 0, err
+	}
+	h := sha256.New()
+	tmp := dst + ".tmp"
+	out, err := os.OpenFile(tmp, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o600)
+	if err != nil {
+		return "", 0, err
+	}
+	size, err := io.Copy(io.MultiWriter(out, h), r)
+	if cerr := out.Close(); err == nil {
+		err = cerr
+	}
+	if err != nil {
+		os.Remove(tmp)
+		return "", 0, err
+	}
+	sha := hex.EncodeToString(h.Sum(nil))
+	// The caller-supplied key must match the content hash; rename to the
+	// canonical "<paste-id>/<sha256>" path so keys are always content-derived.
+	canonical, err := f.path(key[:strings.IndexByte(key, '/')] + "/" + sha)
+	if err != nil {
+		os.Remove(tmp)
+		return "", 0, err
+	}
+	if err := os.MkdirAll(filepath.Dir(canonical), 0o700); err != nil {
+		os.Remove(tmp)
+		return "", 0, err
+	}
+	if err := os.Rename(tmp, canonical); err != nil {
+		os.Remove(tmp)
+		return "", 0, err
+	}
+	return sha, size, nil
+}
+
+func (f *FsBlobStore) Get(key string) (io.ReadSeekCloser, error) {
+	dst, err := f.path(key)
+	if err != nil {
+		return nil, err
+	}
+	file, err := os.Open(dst)
+	if os.IsNotExist(err) {
+		return nil, ErrBlobNotFound
+	}
+	return file, err
+}
+
+func (f *FsBlobStore) Delete(key string) error {
+	dst, err := f.path(key)
+	if err != nil {
+		return err
+	}
+	err = os.Remove(dst)
+	if os.IsNotExist(err) {
+		return ErrBlobNotFound
+	}
+	if err == nil {
+		// best-effort cleanup of the now-empty paste directory
+		os.Remove(filepath.Dir(dst))
+	}
+	return err
+}
+
+func (f *FsBlobStore) Stat(key string) (int64, error) {
+	dst, err := f.path(key)
+	if err != nil {
+		return 0, err
+	}
+	fi, err := os.Stat(dst)
+	if os.IsNotExist(err) {
+		return 0, ErrBlobNotFound
+	}
+	if err != nil {
+		return 0, err
+	}
+	return fi.Size(), nil
+}

internal/store/blob_test.go

@@ -0,0 +1,108 @@
+package store
+
+import (
+	"strings"
+	"testing"
+)
+
+func testBlobs(t *testing.T) *FsBlobStore {
+	t.Helper()
+	b, err := NewFsBlobStore(t.TempDir() + "/files")
+	if err != nil {
+		t.Fatal(err)
+	}
+	return b
+}
+
+func TestBlobPutGetStatDeleteRoundtrip(t *testing.T) {
+	b := testBlobs(t)
+	data := []byte("hello attachment world")
+	sha, size, err := b.Put("abc123/pending", strings.NewReader(string(data)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if size != int64(len(data)) {
+		t.Fatalf("size = %d want %d", size, len(data))
+	}
+	if len(sha) != 64 {
+		t.Fatalf("sha256 = %q", sha)
+	}
+	// canonical key is <paste-id>/<sha256>
+	got, err := b.Get("abc123/" + sha)
+	if err != nil {
+		t.Fatal(err)
+	}
+	buf := make([]byte, len(data)+10)
+	n, _ := got.Read(buf)
+	got.Close()
+	if string(buf[:n]) != string(data) {
+		t.Fatalf("roundtrip mismatch: %q", buf[:n])
+	}
+	sz, err := b.Stat("abc123/" + sha)
+	if err != nil || sz != int64(len(data)) {
+		t.Fatalf("stat = %d, %v", sz, err)
+	}
+	if err := b.Delete("abc123/" + sha); err != nil {
+		t.Fatalf("delete: %v", err)
+	}
+	if _, err := b.Get("abc123/" + sha); err != ErrBlobNotFound {
+		t.Fatalf("get after delete: %v", err)
+	}
+}
+
+func TestBlobStatMissing(t *testing.T) {
+	b := testBlobs(t)
+	if _, err := b.Stat("nope/deadbeef"); err != ErrBlobNotFound {
+		t.Fatalf("want ErrBlobNotFound, got %v", err)
+	}
+}
+
+func TestBlobTraversalPrevention(t *testing.T) {
+	b := testBlobs(t)
+	evil := []string{
+		"../../etc/passwd",
+		"../escape",
+		"..\\windows",
+		"/abs/path",
+		"a/b/c",           // too many segments
+		"onlyone",         // no slash
+		"./relative",      // dot segment
+		"../..",           // bare traversal
+		"ok/../traverse",  // traversal inside
+	}
+	for _, key := range evil {
+		if _, _, err := b.Put(key, strings.NewReader("x")); err == nil {
+			t.Errorf("Put accepted evil key %q", key)
+		}
+		if _, err := b.Get(key); err == nil {
+			t.Errorf("Get accepted evil key %q", key)
+		}
+		if err := b.Delete(key); err == nil {
+			t.Errorf("Delete accepted evil key %q", key)
+		}
+	}
+}
+
+func TestSanitizeFilename(t *testing.T) {
+	cases := [][2]string{
+		{"../../etc/passwd", "passwd"},
+		{"C:\\Users\\evil\\file.txt", "file.txt"},
+		{"normal.txt", "normal.txt"},
+		{"a<b>c", "a<b>c"},
+		{"", "file"},
+		{"..", "file"},
+		{".hidden", ".hidden"},
+		{"with\x00null.txt", "withnull.txt"},
+		{"new\nline.txt", "newline.txt"},
+	}
+	for _, c := range cases {
+		got := SanitizeFilename(c[0])
+		if got != c[1] {
+			t.Errorf("SanitizeFilename(%q) = %q want %q", c[0], got, c[1])
+		}
+	}
+	long := strings.Repeat("x", 300)
+	if got := SanitizeFilename(long); len(got) != MaxFilenameLen {
+		t.Errorf("long name len = %d want %d", len(got), MaxFilenameLen)
+	}
+}

internal/store/store.go

@@ -78,7 +78,8 @@ type CanRow struct {
 }
 
 type Store struct {
-	db *sql.DB
+	db   *sql.DB
+	blob BlobStore // #38: attachment byte storage (nil in some unit tests)
 }
 
 func OpenStore(path string) (*Store, error) {
@@ -95,6 +96,12 @@ func OpenStore(path string) (*Store, error) {
 	if err := s.migrate(); err != nil {
 		return nil, err
 	}
+	// #38: attachment blobs live beside the database under <db>.files
+	blobs, err := NewFsBlobStore(path + ".files")
+	if err != nil {
+		return nil, err
+	}
+	s.blob = blobs
 	return s, nil
 }
 
@@ -142,9 +149,26 @@ deletion_token TEXT
 		last_viewed INTEGER NOT NULL,
 		PRIMARY KEY (paste_id, viewer_id)
 	)`) // #49: per-viewer read dedupe window
+	// #38: file attachments, one per paste in iteration 1. mime is sniffed
+	// server-side before insert; bytes live in the BlobStore keyed
+	// <paste_id>/<sha256>.
+	s.db.Exec(`CREATE TABLE IF NOT EXISTS attachments (
+		id         TEXT PRIMARY KEY,
+		paste_id   TEXT NOT NULL,
+		filename   TEXT NOT NULL,
+		mime       TEXT NOT NULL,
+		size       INTEGER NOT NULL,
+		sha256     TEXT NOT NULL,
+		created_at INTEGER NOT NULL
+	)`)
+	s.db.Exec(`CREATE INDEX IF NOT EXISTS idx_attachments_paste ON attachments(paste_id)`)
 	return err
 }
 
+// Blobs returns the attachment blob store (nil when unavailable, e.g. some
+// unit-test stores).
+func (s *Store) Blobs() BlobStore { return s.blob }
+
 // SlugAlphabet is the paste-id charset (no ambiguous chars).
 var SlugAlphabet = "23456789abcdefghjkmnpqrstuvwxyz"
 

internal/web/static/app.css

@@ -203,6 +203,30 @@ body {
 .iconbtn { border: 1px solid var(--border); background: var(--surface-2); color: var(--muted-fg); border-radius: var(--radius); padding: 5px 12px; font: inherit; font-size: 21.6px; cursor: pointer; text-decoration: none; }
 .iconbtn.gear { display: inline-flex; align-items: center; padding: 5px 9px; }
 .iconbtn.gear svg { width: 22px; height: 22px; }
+/* #127: gear highlights like selected nav tabs; dark toggle button */
+.iconbtn.gear.on { background: var(--accent); color: var(--bg); border-color: var(--accent); }
+.iconbtn.dark-toggle { display: inline-flex; align-items: center; gap: 6px; padding: 5px 9px; }
+.iconbtn.dark-toggle svg { width: 22px; height: 22px; }
+.iconbtn.dark-toggle .icon-moon { display: none; }
+.iconbtn.dark-toggle .icon-sun { display: inline-block; }
+.dark .iconbtn.dark-toggle .icon-sun { display: none; }
+.dark .iconbtn.dark-toggle .icon-moon { display: inline-block; }
+/* #127: gear highlights like selected nav tabs; dark toggle button */
+.iconbtn.gear.on { background: var(--accent); color: var(--bg); border-color: var(--accent); }
+.iconbtn.dark-toggle { display: inline-flex; align-items: center; gap: 6px; padding: 5px 9px; }
+.iconbtn.dark-toggle svg { width: 22px; height: 22px; }
+.iconbtn.dark-toggle .icon-moon { display: none; }
+.iconbtn.dark-toggle .icon-sun { display: inline-block; }
+.dark .iconbtn.dark-toggle .icon-sun { display: none; }
+.dark .iconbtn.dark-toggle .icon-moon { display: inline-block; }
+/* #127: gear highlights like selected nav tabs; dark toggle button */
+.iconbtn.gear.on { background: var(--accent); color: var(--bg); border-color: var(--accent); }
+.iconbtn.dark-toggle { display: inline-flex; align-items: center; gap: 6px; padding: 5px 9px; }
+.iconbtn.dark-toggle svg { width: 22px; height: 22px; }
+.iconbtn.dark-toggle .icon-moon { display: none; }
+.iconbtn.dark-toggle .icon-sun { display: inline-block; }
+.dark .iconbtn.dark-toggle .icon-sun { display: none; }
+.dark .iconbtn.dark-toggle .icon-moon { display: inline-block; }
 .settings-head { padding: 12px 18px; border-bottom: 1px solid var(--border); }
 .settings-head h1 { font-size: 29.2px; font-weight: 600; margin: 0; }
 .settings-body { padding: 16px 18px; color: var(--muted-fg); font-size: 21.6px; }
@@ -480,6 +504,15 @@ a.admin-link:hover { color: var(--fg); text-decoration: underline; }
   .topbar nav a { padding: 5px 8px; font-size: 15px; }
   .iconbtn.gear { padding: 4px 7px; flex-shrink: 0; }
   .iconbtn.gear svg { width: 18px; height: 18px; }
+  .iconbtn.dark-toggle { padding: 4px 7px; flex-shrink: 0; }
+  .iconbtn.dark-toggle svg { width: 18px; height: 18px; }
+  .iconbtn.dark-toggle span { display: none; }
+  .iconbtn.dark-toggle { padding: 4px 7px; flex-shrink: 0; }
+  .iconbtn.dark-toggle svg { width: 18px; height: 18px; }
+  .iconbtn.dark-toggle span { display: none; }
+  .iconbtn.dark-toggle { padding: 4px 7px; flex-shrink: 0; }
+  .iconbtn.dark-toggle svg { width: 18px; height: 18px; }
+  .iconbtn.dark-toggle span { display: none; }
 
   /* new paste: stack editor above sidebar, natural page height */
   .deck {
@@ -627,3 +660,45 @@ button[type="submit"]:focus-visible,
   padding-top: 20px;
   border-top: 1px solid var(--border);
 }
+
+/* #38: attachment upload on /new + attachment display on paste view */
+.side-section .file-input { display: none; }
+.dropzone {
+  border: 1px dashed var(--border);
+  border-radius: var(--radius-lg);
+  padding: 14px 12px;
+  text-align: center;
+  color: var(--muted-fg);
+  cursor: pointer;
+  font-size: 20.7px;
+  transition: border-color .15s, background .15s;
+  background: var(--surface-2, transparent);
+}
+.dropzone:hover, .dropzone.dragover {
+  border-color: var(--accent);
+  background: var(--surface-2);
+}
+.dropzone.dragover { border-width: 2px; }
+.file-chip {
+  display: flex; align-items: center; gap: 10px;
+  border: 1px solid var(--border); border-radius: var(--radius);
+  padding: 8px 14px; margin-top: 8px; background: var(--surface);
+  font-size: 21.6px;
+}
+.file-chip .file-chip-name { overflow: hidden; text-overflow: ellipsis; white-space: nowrap; flex: 1; }
+.file-chip .file-chip-size { color: var(--muted-fg); font-size: 19px; }
+.file-chip .file-chip-remove {
+  border: none; background: none; color: var(--muted-fg); cursor: pointer;
+  font-size: 22px; padding: 0 4px; border-radius: var(--radius-sm);
+}
+.file-chip .file-chip-remove:hover { color: var(--danger, #c0392b); }
+.attachment-bar { display: flex; flex-direction: column; gap: 10px; }
+.attachment-chip {
+  display: inline-flex; align-items: center; gap: 12px; align-self: flex-start;
+  border: 1px solid var(--border); border-radius: var(--radius);
+  padding: 8px 16px; text-decoration: none; color: var(--fg);
+  background: var(--surface-2); font-size: 21.6px;
+}
+.attachment-chip:hover { border-color: var(--accent); }
+.attachment-chip .attachment-size { color: var(--muted-fg); font-size: 19px; }
+.attachment-preview img { max-width: 480px; max-height: 360px; border-radius: var(--radius); border: 1px solid var(--border); }

internal/web/templates/layout.html

@@ -3,11 +3,48 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <link rel="stylesheet" href="/static/app.css">
 <script>
-// preset hook (#16): ?theme= wins; else persisted choice (#100)
+// preset hook (#16): ?theme= wins; else persisted choice (#100).
+// #127: theme pairs - each base theme has a light and dark variant;
+// palette-theme stores the base id, palette-dark the dark flag.
 (function () {
+  var PAIRS = {
+    'midnight':        { light: 'midnight-light',       dark: 'midnight' },
+    'smooth':          { light: 'smooth',               dark: 'smooth-dark' },
+    'pastel-lavender': { light: 'pastel-lavender',      dark: 'pastel-lavender-dark' },
+    'pastel-peach':    { light: 'pastel-peach',         dark: 'pastel-peach-dark' },
+    'pastel-cloud':    { light: 'pastel-cloud',         dark: 'pastel-cloud-dark' }
+  };
+  function resolve(t, dark) {
+    for (var base in PAIRS) {
+      if (t === PAIRS[base].light || t === PAIRS[base].dark) return t;
+      if (t === base) return dark ? PAIRS[base].dark : PAIRS[base].light;
+    }
+    return null;
+  }
   var t = new URLSearchParams(location.search).get('theme');
-  if (!t) try { t = localStorage.getItem('palette-theme'); } catch (e) {}
+  // precedence: URL ?theme= > stored pair+dark prefs > server default dark > dark on
-  if (t) document.documentElement.dataset.preset = t;
+  var dark = {{ defaultDark }};
+  try {
+    var stored = localStorage.getItem('palette-dark');
+    if (stored !== null) dark = stored !== 'false';
+  } catch (e) {}
+  if (t) {
+    // URL ?theme= wins and accepts both base and variant ids; a variant id
+    // also sets the dark flag and normalizes t back to its base pair id.
+    var found = false;
+    for (var base in PAIRS) {
+      if (t === PAIRS[base].dark) { dark = true; t = base; found = true; break; }
+      if (t === PAIRS[base].light) { dark = false; t = base; found = true; break; }
+      if (t === base) { found = true; break; }
+    }
+    if (!found) t = null;
+  }
+  if (!t) {
+    try { t = localStorage.getItem('palette-theme'); } catch (e) {}
+  }
+  if (!PAIRS[t]) t = 'midnight';
+  // do not persist anything here: URL theme is a one-off override
+  document.documentElement.dataset.preset = dark ? PAIRS[t].dark : PAIRS[t].light;
 })();
 </script>
 {{end}}
@@ -22,8 +59,57 @@
       <a href="https://git.archfox.org/poslop/palette" target="_blank" rel="noopener">Git<svg class="ext" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M18 13v6a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2V8a2 2 0 0 1 2-2h6"/><polyline points="15 3 21 3 21 9"/><line x1="10" y1="14" x2="21" y2="3"/></svg></a>
     </nav>
     <div class="spacer"></div>
-    <a class="iconbtn gear" href="/settings" title="Settings" aria-label="Settings">
+    <button class="iconbtn dark-toggle" id="dark-toggle" type="button" title="Toggle dark mode" aria-label="Toggle dark mode">
+      <svg class="icon-sun" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="12" cy="12" r="5"/><line x1="12" y1="1" x2="12" y2="3"/><line x1="12" y1="21" x2="12" y2="23"/><line x1="4.22" y1="4.22" x2="5.64" y2="5.64"/><line x1="18.36" y1="18.36" x2="19.78" y2="19.78"/><line x1="1" y1="12" x2="3" y2="12"/><line x1="21" y1="12" x2="23" y2="12"/><line x1="4.22" y1="19.78" x2="5.64" y2="18.36"/><line x1="18.36" y1="5.64" x2="19.78" y2="4.22"/></svg>
+      <svg class="icon-moon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z"/></svg>
+    </button>
+    <a class="iconbtn gear{{if eq .Page "settings"}} on{{end}}" href="/settings" title="Settings" aria-label="Settings">
       <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="12" cy="12" r="3"/><path d="M19.4 15a1.65 1.65 0 0 0 .33 1.82l.06.06a2 2 0 0 1 0 2.83 2 2 0 0 1-2.83 0l-.06-.06a1.65 1.65 0 0 0-1.82-.33 1.65 1.65 0 0 0-1 1.51V21a2 2 0 0 1-2 2 2 2 0 0 1-2-2v-.09A1.65 1.65 0 0 0 9 19.4a1.65 1.65 0 0 0-1.82.33l-.06.06a2 2 0 0 1-2.83 0 2 2 0 0 1 0-2.83l.06-.06a1.65 1.65 0 0 0 .33-1.82 1.65 1.65 0 0 0-1.51-1H3a2 2 0 0 1-2-2 2 2 0 0 1 2-2h.09A1.65 1.65 0 0 0 4.6 9a1.65 1.65 0 0 0-.33-1.82l-.06-.06a2 2 0 0 1 0-2.83 2 2 0 0 1 2.83 0l.06.06a1.65 1.65 0 0 0 1.82.33H9a1.65 1.65 0 0 0 1-1.51V3a2 2 0 0 1 2-2 2 2 0 0 1 2 2v.09a1.65 1.65 0 0 0 1 1.51 1.65 1.65 0 0 0 1.82-.33l.06-.06a2 2 0 0 1 2.83 0 2 2 0 0 1 0 2.83l-.06.06a1.65 1.65 0 0 0-.33 1.82V9a1.65 1.65 0 0 0 1.51 1H21a2 2 0 0 1 2 2 2 2 0 0 1-2 2h-.09a1.65 1.65 0 0 0-1.51 1z"/></svg>
     </a>
   </div>
+  <script>
+  // #127: dark mode toggle - flips to the other variant of the active pair.
+  (function () {
+    var PAIRS = {
+      'midnight':        { light: 'midnight-light',       dark: 'midnight' },
+      'smooth':          { light: 'smooth',               dark: 'smooth-dark' },
+      'pastel-lavender': { light: 'pastel-lavender',      dark: 'pastel-lavender-dark' },
+      'pastel-peach':    { light: 'pastel-peach',         dark: 'pastel-peach-dark' },
+      'pastel-cloud':    { light: 'pastel-cloud',         dark: 'pastel-cloud-dark' }
+    };
+    var root = document.documentElement;
+    function state() {
+      var p = root.dataset.preset || 'midnight';
+      for (var base in PAIRS) {
+        if (p === PAIRS[base].dark) return { base: base, dark: true };
+        if (p === PAIRS[base].light) return { base: base, dark: false };
+      }
+      return { base: 'midnight', dark: true };
+    }
+    function apply() {
+      var s = state();
+      document.body.classList.toggle('dark', s.dark);
+      root.classList.toggle('dark', s.dark);
+    }
+    function sync(btns) {
+      btns.forEach(function (b) {
+        b.setAttribute('aria-pressed', state().dark ? 'true' : 'false');
+      });
+    }
+    var btns = document.querySelectorAll('.dark-toggle');
+    apply();
+    sync(Array.prototype.slice.call(btns));
+    btns.forEach(function (b) {
+      b.addEventListener('click', function () {
+        var s = state();
+        var dark = !s.dark;
+        root.dataset.preset = dark ? PAIRS[s.base].dark : PAIRS[s.base].light;
+        try { localStorage.setItem('palette-dark', dark ? 'true' : 'false'); } catch (e) {}
+        apply();
+        sync(Array.prototype.slice.call(btns));
+        document.dispatchEvent(new CustomEvent('palette-darkchange'));
+      });
+    });
+  })();
+  </script>
 {{end}}

internal/web/templates/new.html

@@ -63,6 +63,15 @@
         <label class="toggle"><input type="checkbox" id="unlisted"> Unlisted</label>
       </div>
     </div>
+    <div class="float side-section">
+      <h3>Attachment</h3>
+      <div class="dropzone" id="dropzone" title="Paste (Ctrl+V), drop a file here, or click to browse">
+        Drop a file here, press Ctrl+V, or click to browse
+      </div>
+      <input type="file" id="file-input" class="file-input" aria-label="Choose a file">
+      <div id="file-chip-slot"></div>
+      <div class="hint" style="margin-top:6px; font-size:19px; display:none" id="file-text-note">File pastes ignore the text editor (one file per paste)</div>
+    </div>
     <div class="float side-section">
       <h3>Custom URL</h3>
       <input type="text" id="custom" class="custom-input" placeholder="my-snippet">
@@ -180,6 +189,8 @@ const ERROR_MESSAGES = {
   slug_invalid: 'Please keep the Custom URL under 64 characters, using only letters, numbers, dashes, or underscores.',
   content_empty: 'Write or paste something first.',
   content_too_large: 'This paste is too large. The limit is 5 MB.',
+  file_too_large: 'File is too large. The limit is 25 MB.',
+  one_file_only: 'Choose either text or a file for now.',
   expiry_invalid: 'Please pick an expiry between 1 minute and 1 year.',
   rate_limited: 'Too many tries. Wait a minute and try again.',
 };
@@ -250,6 +261,8 @@ content.addEventListener('paste', () => setTimeout(guessLang, 0));
 async function create() {
   // #4: can mode — bundle the editor + extra items into a can via multipart
   if ($('iscan').checked) return createCan();
+  // #38: file attached -> file paste (1 file = 1 paste; text is ignored)
+  if (attachedFile) return createFilePaste();
 
   const body = {
     content: content.value,
@@ -280,6 +293,12 @@ async function create() {
     toast('Create failed', 'error');
     return;
   }
+  finishCreate(data);
+}
+
+// shared success handling for both create paths (#38): result box, copy
+// button, password auto-unlock, then redirect to the paste.
+function finishCreate(data) {
   const url = location.origin + '/' + (data.custom_slug || data.id);
   showResult('<a href="' + url + '">' + url + '</a> <button class="btn btn-icon" id="result-copy" title="Copy URL" type="button">⧉</button>', 'ok');
   $('result').dataset.token = data.deletion_token || '';
@@ -299,7 +318,8 @@ async function create() {
     fd.append('password', $('password').value);
     fd.append('next', dest);
     try {
-      await fetch('/' + data.id, {method: 'POST', body: fd});
+      fetch('/' + data.id, {method: 'POST', body: fd}).finally(() => { location.href = dest; });
+      return;
     } catch(e) {}
   }
   // show the paste
@@ -307,6 +327,115 @@ async function create() {
 }
 $('create').addEventListener('click', create);
 
+// #38: file attachment support — 1 file = 1 paste. Three input paths:
+// Ctrl+V paste of a file, upload button (file picker), drag-and-drop.
+const MAX_FILE_BYTES = 25 * 1024 * 1024;
+let attachedFile = null; // the single attached File, or null
+
+function humanSize(n) {
+  if (n >= 1048576) return (n / 1048576).toFixed(1) + ' MB';
+  if (n >= 1024) return (n / 1024).toFixed(1) + ' KB';
+  return n + ' B';
+}
+
+function setAttachedFile(file) {
+  if (!file) return clearAttachedFile();
+  if (file.size > MAX_FILE_BYTES) {
+    toast('File is too large. The limit is 25 MB.', 'error');
+    return;
+  }
+  attachedFile = file;
+  renderFileChip();
+  $('file-text-note').style.display = 'block';
+}
+
+function clearAttachedFile() {
+  attachedFile = null;
+  $('file-input').value = '';
+  renderFileChip();
+  $('file-text-note').style.display = 'none';
+}
+
+function renderFileChip() {
+  const slot = $('file-chip-slot');
+  slot.innerHTML = '';
+  if (!attachedFile) return;
+  const chip = document.createElement('div');
+  chip.className = 'file-chip';
+  chip.innerHTML = '<span class="file-chip-name"></span>' +
+    '<span class="file-chip-size">' + humanSize(attachedFile.size) + '</span>' +
+    '<button type="button" class="file-chip-remove" title="Remove file">&times;</button>';
+  chip.querySelector('.file-chip-name').textContent = attachedFile.name;
+  chip.querySelector('.file-chip-remove').addEventListener('click', clearAttachedFile);
+  slot.appendChild(chip);
+}
+
+// (1) file picker via the dropzone click
+$('dropzone').addEventListener('click', () => $('file-input').click());
+$('file-input').addEventListener('change', () => {
+  if ($('file-input').files.length) setAttachedFile($('file-input').files[0]);
+});
+
+// (2) drag-and-drop onto the dropzone (and the page broadly)
+const dz = $('dropzone');
+['dragenter', 'dragover'].forEach(ev => {
+  document.addEventListener(ev, e => {
+    if (!e.dataTransfer || ![...e.dataTransfer.types].includes('Files')) return;
+    e.preventDefault();
+    dz.classList.add('dragover');
+  });
+});
+['dragleave', 'drop'].forEach(ev => {
+  document.addEventListener(ev, e => {
+    if (ev === 'drop') e.preventDefault();
+    if (e.target === dz || ev === 'drop') dz.classList.remove('dragover');
+  });
+});
+document.addEventListener('drop', e => {
+  if (!e.dataTransfer || !e.dataTransfer.files.length) return;
+  e.preventDefault();
+  setAttachedFile(e.dataTransfer.files[0]);
+});
+
+// (3) Ctrl+V of a file anywhere on the page
+document.addEventListener('paste', e => {
+  const files = e.clipboardData && e.clipboardData.files;
+  if (!files || !files.length) return; // normal text paste into the editor
+  e.preventDefault();
+  setAttachedFile(files[0]);
+});
+
+// file create path: POST multipart. 1 file = 1 paste: when a file is
+// attached the editor text is ignored (server enforces this too).
+async function createFilePaste() {
+  const fd = new FormData();
+  fd.append('file', attachedFile);
+  if ($('title').value) fd.append('title', $('title').value);
+  if ($('custom').value) fd.append('custom_slug', $('custom').value);
+  if ($('haspw').checked) fd.append('password', $('password').value);
+  if ($('burn').checked) {
+    fd.append('burn_after_read', 'true');
+    fd.append('burn_after_reads', String(parseInt($('burnreads').value, 10) || 1));
+  }
+  if ($('unlisted').checked) fd.append('visibility', 'unlisted');
+  const exp = document.querySelector('input[name="exp"]:checked').value;
+  if (exp === 'custom') {
+    const dur = composeCustomExpiry();
+    if (dur === null) { toast('Check the custom expiry', 'error'); return; }
+    fd.append('expires_in', dur);
+  } else if (exp) {
+    fd.append('expires_in', exp);
+  }
+  const res = await fetch('/api/pastes', { method: 'POST', body: fd });
+  const data = await res.json();
+  if (!res.ok) {
+    showResult(friendlyError(data), 'err');
+    toast('Create failed', 'error');
+    return;
+  }
+  finishCreate(data);
+}
+
 // #4: can creation — POST multipart to /api/pastes/can. The main editor is
 // the first item; each extra can-item row is another text item.
 async function createCan() {

internal/web/templates/paste.html

@@ -40,6 +40,19 @@
     </div>
   </div>
   {{end}}
+  {{if .Attachment}}
+  <div class="float">
+    <div class="attachment-bar">
+      <a class="attachment-chip" href="/f/{{.Attachment.ID}}/{{.Attachment.Filename}}" data-mime="{{.Attachment.Mime}}">
+        <span class="attachment-name">{{.Attachment.Filename}}</span>
+        <span class="attachment-size">{{.Attachment.SizeHuman}}</span>
+      </a>
+      {{if or (eq .Attachment.Mime "image/png") (eq .Attachment.Mime "image/jpeg") (eq .Attachment.Mime "image/gif") (eq .Attachment.Mime "image/webp")}}
+      <div class="attachment-preview"><img src="/f/{{.Attachment.ID}}/{{.Attachment.Filename}}" alt="{{.Attachment.Filename}}"></div>
+      {{end}}
+    </div>
+  </div>
+  {{end}}
   <div class="float">
     <div class="code"><div class="gutter">{{.Gutter}}</div><div class="codebody" id="codebody">{{.ContentHTML}}</div></div>
   </div>

internal/web/templates/settings.html

@@ -8,6 +8,11 @@
     <div class="settings-body">
       <h3>Theme</h3>
       <div class="theme-grid" id="theme-grid"></div>
+      <button class="iconbtn dark-toggle" id="settings-dark-toggle" type="button" title="Toggle dark mode" aria-label="Toggle dark mode">
+        <svg class="icon-sun" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><circle cx="12" cy="12" r="5"/><line x1="12" y1="1" x2="12" y2="3"/><line x1="12" y1="21" x2="12" y2="23"/><line x1="4.22" y1="4.22" x2="5.64" y2="5.64"/><line x1="18.36" y1="18.36" x2="19.78" y2="19.78"/><line x1="1" y1="12" x2="3" y2="12"/><line x1="21" y1="12" x2="23" y2="12"/><line x1="4.22" y1="19.78" x2="5.64" y2="18.36"/><line x1="18.36" y1="5.64" x2="19.78" y2="4.22"/></svg>
+        <svg class="icon-moon" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z"/></svg>
+        <span>Dark mode</span>
+      </button>
       <p class="admin-link-row"><a class="admin-link" href="/admin">Admin</a></p>
     </div>
   </div>
@@ -16,17 +21,13 @@
 (function () {
   // #112: derive each preset's swatches from the real CSS variables in
   // app.css by temporarily applying data-preset, so they can never drift.
-  var themeNames = [
+  // #127: 5 theme pairs, light swatches top row, dark bottom row.
+  var pairs = [
     { id: 'midnight',        name: 'Midnight' },
     { id: 'smooth',          name: 'Smooth' },
     { id: 'pastel-lavender', name: 'Pastel Lavender' },
     { id: 'pastel-peach',    name: 'Pastel Peach' },
-    { id: 'pastel-cloud',    name: 'Pastel Cloud' },
+    { id: 'pastel-cloud',    name: 'Pastel Cloud' }
-    { id: 'midnight-light',       name: 'Midnight Light' },
-    { id: 'smooth-dark',          name: 'Smooth Dark' },
-    { id: 'pastel-lavender-dark', name: 'Pastel Lavender Dark' },
-    { id: 'pastel-peach-dark',    name: 'Pastel Peach Dark' },
-    { id: 'pastel-cloud-dark',    name: 'Pastel Cloud Dark' }
   ];
   var SWATCH_VARS = ['--bg', '--surface', '--surface-2', '--muted', '--accent'];
 
@@ -40,26 +41,60 @@
     return colors;
   }
 
+  // current base pair + dark flag from the resolved data-preset.
+  // "midnight" is itself the dark variant, so check dark ids first.
+  function state() {
+    var p = document.documentElement.dataset.preset || 'midnight';
+    if (p === 'midnight' || /-dark$/.test(p)) {
+      return { base: p === 'midnight' ? 'midnight' : p.replace(/-dark$/, ''), dark: true };
+    }
+    return { base: p === 'midnight-light' ? 'midnight' : p, dark: false };
+  }
+
   var grid = document.getElementById('theme-grid');
-  var current = document.documentElement.dataset.preset || 'midnight';
+  var cards = {};
-  themeNames.forEach(function (t) {
+  pairs.forEach(function (t) {
-    var colors = presetColors(t.id);
+    var light = presetColors(t.id);
+    var dark = presetColors(t.id + '-dark');
     var btn = document.createElement('button');
     btn.type = 'button';
     btn.className = 'theme-card';
-    btn.setAttribute('aria-pressed', current === t.id ? 'true' : 'false');
+    btn.setAttribute('aria-pressed', 'false');
+    btn.setAttribute('data-pair', t.id);
     btn.innerHTML = '<strong>' + t.name + '</strong>' +
-      '<span class="swatches">' + colors.map(function (c) {
+      '<span class="swatches">' + light.map(function (c) {
+        return '<span class="swatch" style="background:' + c + '"></span>';
+      }).join('') + '</span>' +
+      '<span class="swatches">' + dark.map(function (c) {
         return '<span class="swatch" style="background:' + c + '"></span>';
       }).join('') + '</span>';
     btn.addEventListener('click', function () {
-      document.documentElement.dataset.preset = t.id;
+      var dark = state().dark;
+      document.documentElement.dataset.preset = dark ? t.id + '-dark' : t.id;
       try { localStorage.setItem('palette-theme', t.id); } catch (e) {}
-      grid.querySelectorAll('.theme-card').forEach(function (c) { c.setAttribute('aria-pressed', 'false'); });
+      Object.keys(cards).forEach(function (k) { cards[k].setAttribute('aria-pressed', 'false'); });
       btn.setAttribute('aria-pressed', 'true');
     });
+    cards[t.id] = btn;
     grid.appendChild(btn);
   });
+
+  function sync() {
+    var s = state();
+    Object.keys(cards).forEach(function (k) {
+      cards[k].setAttribute('aria-pressed', k === s.base ? 'true' : 'false');
+    });
+    var dt = document.getElementById('settings-dark-toggle');
+    if (dt) dt.setAttribute('aria-pressed', s.dark ? 'true' : 'false');
+  }
+  sync();
+  // the topbar script runs before this button exists, so wire it here
+  var dt = document.getElementById('settings-dark-toggle');
+  dt.addEventListener('click', function () {
+    var btns = document.querySelectorAll('.topbar .dark-toggle');
+    if (btns.length) btns[0].click(); else document.dispatchEvent(new CustomEvent('palette-darkchange'));
+  });
+  document.addEventListener('palette-darkchange', sync);
 })();
 </script>
 {{template "foot" .}}

internal/web/web.go

@@ -24,6 +24,44 @@ import (
 //go:embed templates/*.html
 var tmplFS embed.FS
 
+// DefaultDark is the server-configured default dark mode state for visitors
+// without stored preferences (#127). Set via PALETTE_DEFAULT_DARK.
+var DefaultDark = true
+
+// SetDefaultDark applies the PALETTE_DEFAULT_DARK env var: unset = dark on,
+// "false"/"0"/"off" = dark off.
+func SetDefaultDark(v string) {
+	switch strings.ToLower(strings.TrimSpace(v)) {
+	case "false", "0", "off":
+		DefaultDark = false
+	default:
+		DefaultDark = true
+	}
+}
+
+// ResolvePreset maps a theme pair id (or explicit variant id) plus dark state
+// to a concrete preset id. Returns the defaults when pair is empty or unknown.
+// Precedence handled by callers: URL ?theme= > stored pair+dark > defaults.
+func ResolvePreset(pair string, dark bool) string {
+	variants := map[string][2]string{ // pair -> {light, dark}
+		"midnight":        {"midnight-light", "midnight"},
+		"smooth":          {"smooth", "smooth-dark"},
+		"pastel-lavender": {"pastel-lavender", "pastel-lavender-dark"},
+		"pastel-peach":    {"pastel-peach", "pastel-peach-dark"},
+		"pastel-cloud":    {"pastel-cloud", "pastel-cloud-dark"},
+	}
+	if v, ok := variants[pair]; ok {
+		if dark {
+			return v[1]
+		}
+		return v[0]
+	}
+	if dark {
+		return "midnight"
+	}
+	return "midnight-light"
+}
+
 //go:embed static
 var staticFS embed.FS
 
@@ -33,8 +71,9 @@ type UI struct {
 
 func New() (*UI, error) {
 	funcs := template.FuncMap{
-		"humanSize": humanSize,
+		"humanSize":   humanSize,
-		"version":   func() string { return Version }, // #93: topbar version label
+		"version":     func() string { return Version },   // #93: topbar version label
+		"defaultDark": func() bool { return DefaultDark }, // #127: server-configured default
 	}
 	t, err := template.New("").Funcs(funcs).ParseFS(tmplFS, "templates/*.html")
 	if err != nil {

internal/web/web_test.go

@@ -0,0 +1,48 @@
+package web
+
+import "testing"
+
+func TestResolvePreset(t *testing.T) {
+	cases := []struct {
+		pair string
+		dark bool
+		want string
+	}{
+		{"", false, "midnight-light"},
+		{"", true, "midnight"},
+		{"midnight", false, "midnight-light"},
+		{"midnight", true, "midnight"},
+		{"smooth", false, "smooth"},
+		{"smooth", true, "smooth-dark"},
+		{"pastel-lavender", true, "pastel-lavender-dark"},
+		{"pastel-peach", false, "pastel-peach"},
+		{"pastel-cloud", true, "pastel-cloud-dark"},
+		{"bogus", true, "midnight"},
+		{"bogus", false, "midnight-light"},
+		// explicit variant ids also resolve (URL ?theme= may pass them)
+		{"smooth-dark", true, "midnight"},
+	}
+	for _, c := range cases {
+		if got := ResolvePreset(c.pair, c.dark); got != c.want {
+			t.Errorf("ResolvePreset(%q, %v) = %q, want %q", c.pair, c.dark, got, c.want)
+		}
+	}
+}
+
+func TestSetDefaultDark(t *testing.T) {
+	for _, off := range []string{"false", "0", "off", "OFF", " false "} {
+		DefaultDark = true
+		SetDefaultDark(off)
+		if DefaultDark {
+			t.Errorf("SetDefaultDark(%q): want dark off", off)
+		}
+	}
+	for _, on := range []string{"true", "1", "on", "anything"} {
+		DefaultDark = false
+		SetDefaultDark(on)
+		if !DefaultDark {
+			t.Errorf("SetDefaultDark(%q): want dark on", on)
+		}
+	}
+	DefaultDark = true
+}