Open Files > Advanced Search. There is a checkbox labelled Show only manually uploaded files. What does this mean?
Topic on Talk:Phabricator/Help
I'm sorry to say that it is not entirely clear to me either, as Phabricator's docs only say "isExplicitUpload: Used to show users files they explicitly uploaded." It might have to do with access restrictions, e.g. if other users can access your image or not.