How to make thumbnail generator process files with .divx extension?

I am working on modifying G2 to use the DIVX player for DIVX files.
I have proof-of-concept working, but it seems that with the way things work, the only way to allow it to distinguish between a DIVX and other kinds of AVI is to upload DIVX files with the .divx extension (Well that isn't true, what I mean is I would not want to hack AVIs so that it always tries to use the DIVX player - I want it to use normal associated player - ie WMPlayer - for most AVIs, but the DIVX player for DIVX files)

I have managed to mod G2 to use the DIVX web player for .divx files, but I cannot work out yet how to persuade the thumbnail generator to try and create thumbnails for files with extension .divx

If I upload a DIVX with the extension .AVI, it makes the thumbnail fine using ffmpeg, proving that ffmpeg is capable of making thumbnails for the file. However, if I take the same file and give it the extension .divx, the thumbnail generator does not seem to try and generate thumbnails at all.

What files do I need to be looking in? I am a complete G2 newb, so am a bit lost.

TIA