Files need to be tagged properly in order to be shown.
Since you do not have proper metadata in them, they will not be shown. Tags are only imported from the mp3 files themselves once, if there are none, UpdateMedia will not ask for them again...
You will need to go to the web admin, and use Files & Media > Media/Files sync, to add tags to all your music, so that it will be viewable. Go ahead and attach the cover art there too.