You cannot comment on this file
Posting a comment on a file download page returned that you cannot comment on this file (403).
Why this happens
assertFileViewableOnDownloadPage failed: the file is effectively private and the commenter is not the owner. Distinct from Comments are disabled (feature off) and from empty/captcha/rate-limit errors.
Diagnose and fix
1. Owner: set the file Public if guests/members should comment.
2. Commenter: open a public file page, or sign in as the owner.
3. Admin: check public override on the user/file if staff need visibility.
4. Confirm Enable file comments is on — otherwise you see comments disabled instead.