Thanks for working on this.  Could you please post the fix as a patch file?  If you don't have mercurial, you can generate the diff on windows using the python diff module (scripts/diff.py -u <yourfile> <origfile>).  Actually, I'm not sure exactly where diff is in the windows install, but I know it is there.

Do you know if image/x-png and image/png are included in the registry on all windows versions?  If so we could use that key for a unit test.