MP4 is the world's default video: the file that plays before anyone has to think about it. It was born in 2001, the MPEG standards group, built on Apple's QuickTime design, and its real weak spot is very little day to day: as a container its quality depends on the codec inside.
Video is the heaviest thing most of us ever attach: screen recordings, camera clips, whole talks. Stands in for any video format Flippy can read: MP4, MOV, MKV, WEBM and the rest. That makes it the right destination when what you need is any moving picture: recordings, rips, exports, downloads.