fileName now take the upload code

README.md

@@ -270,10 +270,12 @@ paths or stack traces.
   (salted), never in plaintext. Successful entry mints a short-lived HMAC-signed
   cookie, so access is granted by the server and cannot be forged.
 - **Path safety.** Filesystem paths are built only from server-generated,
-  regex-validated ids (`^[a-z0-9]{12}$`). The original filename is metadata
+  regex-validated ids (`^[a-z0-9]{8}$`). The original filename is metadata
   only and is sanitized (basename, control/null bytes stripped) before being
-  stored or emitted in `Content-Disposition`. Path traversal, `../`, encoded
+  stored; downloads are sent as `<upload-key>.<extension>`, so the basename in
-  traversal, null bytes, and symlink injection via user input are not possible.
+  `Content-Disposition` is always the server-generated key. Path traversal,
+  `../`, encoded traversal, null bytes, and symlink injection via user input are
+  not possible.
   Downloads additionally refuse symlinks (`lstat`) as defense in depth.
 - **MIME / XSS.** Downloads are sent as `Content-Disposition: attachment` (browsers
   download rather than render) with `X-Content-Type-Options: nosniff`, so a spoofed

src/routes/download.ts

@@ -2,7 +2,7 @@ import { lstat } from "node:fs/promises";
 import { hasValidAuth, renderPasswordForm } from "../auth";
 import { notFound } from "../http";
 import { log } from "../log";
-import { contentDisposition, fileIdFromKey, KEY_PATTERN, secretMatches } from "../security";
+import { contentDisposition, downloadFileName, fileIdFromKey, KEY_PATTERN, secretMatches } from "../security";
 import { isExpired, type Storage } from "../storage";
 
 export async function handleDownload(
@@ -41,7 +41,7 @@ export async function handleDownload(
   const headers = new Headers({
     "content-type": meta.mimeType || "application/octet-stream",
     "content-length": String(fileStat.size),
-    "content-disposition": contentDisposition(meta.originalName),
+    "content-disposition": contentDisposition(downloadFileName(rawKey, meta.originalName)),
     "cache-control": "private, no-store",
     "x-content-type-options": "nosniff",
     "referrer-policy": "no-referrer",

src/security.ts

@@ -59,6 +59,12 @@ export function contentDisposition(name: string): string {
   return `attachment; filename="${ascii}"; filename*=UTF-8''${encoded}`;
 }
 
+export function downloadFileName(key: string, originalName: string): string {
+  const dot = originalName.lastIndexOf(".");
+  const ext = dot > 0 && dot < originalName.length - 1 ? originalName.slice(dot + 1) : "";
+  return ext ? `${key}.${ext}` : key;
+}
+
 export function hashPassword(password: string): string {
   const salt = randomBytes(16).toString("hex");
   const hash = scryptSync(password, salt, 64).toString("hex");