When I select "Unused file extensions" and analyze, the program lists completely valid and used extensions as unused.

The ones I refer to is located at HKCU\Software\Microsoft\Windows\Currentversion\Explorer\FileExts\

In this particular case they are in the usual format, but instead of using subkeys it has a stringvalue named "ProgId" with the value "Xnview.Image".

These are completely valid, and are used, a doubleclick starts the program etc.

just my suggestion

fix it "fix selected issues" .. and save a backup .. see if anything screws up! .. if it does .. revert back ... if it doesnt .. well ..

I know, and I do.

When it says extensions defined with only a progid string are unused, and that mechanism is one of the "legal" ones for associating files to a program, then in my opinion this is a bug.