#281: /raw streams attachment blob for all attachment mimes

handleRaw only streamed the blob behind an isImageMime gate (#221), so
non-image attachment pastes fell through to empty row.Content and /raw
served 0 bytes. Serve the blob for every attachment mime, passing the
sniffed mime through serveContentType so active-content types (html,
svg, xml) still serve as text/plain per the #34 rule. Regression tests
cover text and html attachments (size, Content-Type, byte equality).

internal/api/attachments.go

@@ -91,17 +91,6 @@ func (l *limitReader) Read(p []byte) (int, error) {
 	return n, err
 }
 
-// isImageMime reports whether the sniffed mime is a raster image the viewer
-// can render inline (#221). SVG is excluded: it is forced to text/plain on
-// serving by the active-content rule and must never render as an image.
-func isImageMime(mime string) bool {
-	switch mime {
-	case "image/png", "image/jpeg", "image/gif", "image/webp":
-		return true
-	}
-	return false
-}
-
 // 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