As used by Honk and supported by Mastodon[1]
Related thread: https://honk.tedunangst.com/u/tedu/h/4M3ps7Lc1yXR57mJNL
1: https://github.com/mastodon/mastodon/blob/684a970b3c7ce19ec419c4db52b67a3b0903f493/app/lib/activitypub/parser/media_attachment_parser.rb#L30