Co-authored-by: emilweth <7402764+emilweth@users.noreply.github.com>

* Fix attachments getting processed despite failing content-type validation

* Add a restrictive ImageMagick security policy tailored for Mastodon

* Fix misdetection of MP3 files with large cover art

* Reject unprocessable audio/video files instead of keeping them unchanged

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

Change wording of “Content cache retention period” setting to highlight destructive implications (#23261)