I happily found this
pet@pet-desktop:~/Desktop/python$ file mit-6-00-f08-lec01_300k.mp4 mit-6-00-f08-lec01_300k.mp4: ISO Media, MPEG v4 system, version 2 pet@pet-desktop:~/Desktop/python$ file ../berckly.flv ../berckly.flv: ISO Media, MPEG v4 system, version 2 pet@pet-desktop:~/Desktop/python$they are same format, though the video files downloaded form berkeley has, defualtly, no extention