windows media center question.

Basicly i have converted all my video files into mpeg 4 so i could put them on a usb and watch them on my xbox, but now i have gotten media center to work on my 360 and when i go into videos it says i have non videos yet i have over 50 videos but there mpeg4 not avi, so i was wandering can u actually watch mpeg 4 videos in windows media center ot do they have to be avi ??