v3: comments say what the code can't — the documentation pass wanted JSDoc on every export and every field of an exported interface, which filled the code with restatements ("The dump.", "Whether it is loading.", "The props of X.", "Renders the X.") and module docs that told a reader nothing ("The hot and new feeds, and the props every main feed of the index takes."); some were wrong as well as vague. Every comment in api/, shared/ and src/ is rewritten under one rule, now stated in the README's "Comments" section: every module and export still has a JSDoc, and it gives context (who uses it and where), usage (what a caller must know: a precondition, what it leaves to the caller, a side effect) or its place in the chain (where its input comes from, where its output goes); a field, parameter, private function or local gets one only when it adds to its parent's, and context about a whole type goes on the type; a type's doc is phrased from the value's side ("Returned to Y by X when Z", "Passed to Y by X"); full sentences, as short as they can be, no label-colon-list; // goes wherever the code isn't obvious and says why, and the section banners and narration go. Every claim was checked against the code, which turned up some forty comments that were wrong: DumpCard's claimed it was only used by the hot and new feeds, the WebSocket welcome claimed to be a socket's first message (presence comes first), UpdateUserRequest said null removes a description (it keeps it), deletedDumpIds leaves out that it also holds dumps made private, THEME_COLOR claimed to mirror index.html's, and the like. The routers' JSDoc now says where they are mounted and what becomes of a thrown APIException. doc-lint.ts lets a field without a doc through; a module or export without one still fails. No code changes; 204 backend and 628 frontend tests pass, and doc:lint, deno check, deno lint and tsc are clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/lib/upload.ts

@@ -1,5 +1,6 @@
 /**
- * Checking uploaded images and serving uploaded files back.
+ * Checks the images users upload and serves them back. The avatar, playlist
+ * cover, dump thumbnail and attachment routes call it.
  *
  * @module
  */
@@ -29,7 +30,8 @@ export {
 
 /**
  * Returns the MIME type of a JPEG, PNG, GIF or WebP image from its magic
- * bytes, ignoring the type the browser declared; null for anything else.
+ * bytes, or null for anything else. The type the browser declared is never
+ * trusted, since the file is later served with the type found here.
  */
 export function detectImageMime(data: Uint8Array): string | null {
   if (data[0] === 0xFF && data[1] === 0xD8 && data[2] === 0xFF) {
@@ -52,7 +54,8 @@ export function detectImageMime(data: Uint8Array): string | null {
 }
 
 /**
- * Checks an uploaded image's size and content, and returns its MIME type.
+ * Checks an uploaded image's size and content, and returns its MIME type,
+ * which the route stores to serve the image with.
  *
  * @throws {APIException} 400 when it exceeds {@link MAX_IMAGE_SIZE_BYTES} or
  * isn't a JPEG, PNG, GIF or WebP image.
@@ -77,9 +80,10 @@ export function validateImageUpload(data: Uint8Array): string {
 }
 
 /**
- * Serves an uploaded file inline as `mime`, cached for `cacheMaxAge` seconds
- * (privately when `isPrivate`). Answers 404 and resolves false when the file
- * is missing.
+ * Serves an uploaded image inline as `mime`, cached for `cacheMaxAge` seconds,
+ * and only by the browser when `isPrivate`. It answers 404 and resolves false
+ * when the file is missing. The caller checks first that the viewer may see
+ * it.
  */
 export async function serveUploadedFile(
   ctx: Context,
@@ -89,6 +93,8 @@ export async function serveUploadedFile(
   isPrivate = false,
 ): Promise<boolean> {
   ctx.response.headers.set("Content-Disposition", "inline");
+  // The browser must take the checked type, never guess another from the
+  // bytes, and a frontend on another origin may still show the image.
   ctx.response.headers.set("X-Content-Type-Options", "nosniff");
   ctx.response.headers.set("Cross-Origin-Resource-Policy", "cross-origin");
   const served = await serveFile(ctx, filePath, {