Cap attachment filenames at 128 chars server-side

A 250-char multipart filename was accepted and echoed verbatim in
Content-Disposition. SanitizeFilename already truncates; lower the cap
from 255 to 128 so DB rows and header echoes stay bounded (#248).

internal/store/blob_test.go

@@ -105,4 +105,9 @@ func TestSanitizeFilename(t *testing.T) {
 	if got := SanitizeFilename(long); len(got) != MaxFilenameLen {
 		t.Errorf("long name len = %d want %d", len(got), MaxFilenameLen)
 	}
+	// issue #248: a 250-char multipart filename must truncate to the cap
+	repro := strings.Repeat("b", 246) + ".txt"
+	if got := SanitizeFilename(repro); len(got) != MaxFilenameLen {
+		t.Errorf("repro name len = %d want %d", len(got), MaxFilenameLen)
+	}
 }