I added a diff file to the bug.
Dunno if that's the same as a patch file, or how to create a patchfile if it's not.

>Do you know if image/x-png and image/png are included in the registry on all 
> windows versions?

I think your question is reversed, in the same way that the code was reversed.
You're not looking for image/png and/or image/x-png. You're looking for .png in order to retrieve its mimetype (aka Content Type).
While nothing is 100% certain on Windows :), I'm quite confident that every copy will have an HKCR\.png regkey, and that regkey will have a Content Type value, and that value's setting will be the appropriate mometype, which I'd expect to be image/png.

I was kinda surprised to find this bug as it's so obvious
I started chasing it because Chrome kept complaining that pngs were being served as image/x-png (by CherryPy).
There are other bugs (eg: 15199, 10551) that my patch should fix.

-Dave