Change WordPress user roles and capabilities › Forums › Restrict or Permit access inside WordPress – how to › Post Edit Access: Granting other users access to uploaded media files › Reply To: Post Edit Access: Granting other users access to uploaded media files
Thanks Vladimir, that explains it. Looks like the key thing is ensuring the media file is attached to a page. Put simply, the feature works for attached files, but not designed to apply to unattached files. This works for my use-case, cheers.