When you try to save a webp image using our Save As dialog, the type is "Firefox HTML Document".

It should be WEBP image, similar to JPG/PNG.

Go to this image and try to save it:

https://s.yimg.com/uu/api/res/1.2/qG4H6QhbSmQl.7LqNfIGZg--~B/Zmk9c3RyaW07aD0xNjA7cT04MDt3PTM0MDthcHBpZD15dGFjaHlvbg--/https://s.yimg.com/os/creatr-uploaded-images/2020-08/9bd1e9c0-d6ed-11ea-9def-c80473c346ae.cf.webp

On macOS the dropdown says "Document" but the file extension says webp. What OS are you testing on and what, exactly, doesn't work?

Windows.

It gets the correct extension when we save, so this is only about the text in the dialog. I'll update the bug.

Molly, is this something left over after bug 1570477 ? Can we change the description of the other filetypes we handle in the registry so we don't pretend they're all HTML docs?

That string comes from the file type registration (which in this case was created by our installer) unless the application that opened the dialog overrides it using a filter string (of the kind that we generated in this function for saving web pages). We probably shouldn't generated our own filter strings for every file type we try to save, so that brings us to changing what the installer puts in the registry.
Right now (as you can guess from this bug) we have one description for every file type, and it would be a major pain to start having separate ones (we'd have to make a bunch of new handler entries, one for each type, which also means migration code, and new uninstall logic, and ...), but even having just one string there's no reason it should say "HTML" in it now that we're using it for other things.

So I'm moving this bug to Installer and making it about changing that description string.

tbf I'd also be OK with having the mime info code overwrite some of these descriptions with generic ones. As it is, if you have chrome installed and that registered the filetype first, we'll say "Chrome HTML document" which of course is no better...

Still, because other apps will also use this description, updating our own to not use "HTML" for things that, well, aren't, is probably a good idea, so I'll file a separate bug.