I tried uploading a movie in MPEG 4 format with the extension .mp4, but
when I linked to it with [[media:movie_name.mp4]] it just came up as
gibberish, which usually would mean that the mime time was wrong. I'm
pretty sure that giving it a .mov extension would work fine so long as
the user has Quicktime 6, but that's not really as correct as mp4. (And
there are probably players out there that can handle MPEG 4 but not
quicktime movies in general.)
Nathan