It has not been well described so I decided to publish what I got from them.

"The format for acceptable codecs has changed to the following effective
October 31, 2008. Please adhere to these formats going forward.

Acceptable codecs:

0x00000000 0x10000001 0x10000002
cvid
dvsd DVSd dvc dvcp
MJPG AVRn ADVJ jpeg mjpa mjpb
DX50 DIVX MP42 FMP4 MP43 avc1 MPG4 M4S2 mp4v
rle
XVID

If you attempt to upload a format such as AVI (which was once acceptable
before) you will not be able to do so any longer."