This file type is not allowed on your account
Copy from public refused the source because its MIME/filename fails isMimeAllowed — same safety idea as This file type is not allowed, with account-framed wording on the copy path only.
Why this happens
The public file’s type is on the hardcoded deny list (or fails the MIME check). Site extension allow/deny may still apply on the next check with the shorter “not allowed” / “blocked” messages.
Diagnose and fix
1. Same as site policy: Allowed / Blocked file extensions under Settings → Files.
2. If direct upload of a similar file works but copy fails, compare the source file’s real name/MIME in admin.
3. Member: copy a different public file, or download and re-upload only if policy allows that type.
4. Admins should not weaken dangerous-type blocks just to unblock one copy.