I struggled a bit trying to figure this out. A lot of information out there is inaccurate or doesn’t apply to my environment.

If you’re in a similar situation where you want to associate a certain command with a certain filetype, you probably don’t need to bother with “progids” or whatever else is recommended out there. It didn’t work for me.

What currently works for me is adding keys under Computer\HKLM\SOFTWARE\Classes\SystemFileAssociations<extension>

And some pictures for clarity, adding VComp to .mp4 files:

(The values in here are not important. I did not create those.)

There are also ways to add sub-command menus via the registry. Some resources will tell you that’s impossible, but that’s not true. I can’t remember exactly how I did it, but here is a screenshot with some proof and a link to a helpful page.