All files downloaded directly from the Music and Movies app for offline use are encrypted and can only be used inside said apps. You can browse for the files but they won't be an MP3 or MP4/MOV/AVI file. For Movies, there's no going around that. For Music, however, you can download music you own (not streaming ones if you have an All Access/YouTube Red subscription) on your computer in MP3 format and then copy that over to your device (also works directly on your device if you use the Music web page, not the app).