Maybe it is not a divx film, but someone said it was one.
I hope this utility exists on Mac as well: Open a terminal and execute file filename
tux@ubuntu:~$ file nobody*
nobody-needs-to-know.avi: RIFF (little-endian) data, AVI, 640 x 352, 23.98 fps, video: XviD, audio: MPEG-1 Layer 3 (stereo, 48000 Hz)
nobody-needs-to-know.txt: Non-ISO extended-ASCII English text, with CRLF line terminators