Add additional filetype to a specific “Kind”

I would say the best way to do this would be some sort of script that searches your drive on a scheduled basis, and dumps the results into a text file for you.

Something like:

sudo find / -name "*.avi" -o -name "*.mp4" -o -name "*.m4v" -o -name "*.mp3" > ~/Dropbox/results.txt

The -o switch is the find equivalent of logical OR, so the above is searching the whole of your computer for any files (or directories) that have the extensions .avi, .mp4, .m4v, or .mp3

Short answer: I don't think you can change or add to the defined groups. As best I can tell groups are defined on the system level, based on Apple-declared uniform type identifiers (UTI). I suppose one could (conceivably) add new UTIs by creating an app that declares them and propagates them to the system, but that seems like overkill.

However, there are a couple of options you might try:

• Group by Application: This will group all files together by the default application that opens them. It's more fine-grained than Kind, but depending on how your application defaults are set up it might not produce the results you want.
• Group by Tags: This will involve some prep-work setting up Folder Action scripts that will tag your files according to your desired criteria, but this gives you fine-grained control over how files are grouped in Finder windows.