Powertoys now supports a thumbnail handler for the Quite OK Image format (QOI). Preview appears to work as well. It's a little slow but does the job on both Explorer and DOpus. Leo already posted the link in the SVG thread but here it is for convenience:
For those only interested in the thumbnails, a standalone QOI thumbnail handler is here:
And more information about the QOI image format can be found here:
There's now SVG shell extensions other than that supplied with PowerToys, with preview and thumbnail. It seems to be (for me at least) quite a bit faster than the PowerToys version.
Finally, RECOIL has a thumb and preview handler. RECOIL ( Retro Computer Image Library) can open 551 retro image fileformats. I suspect most are just too old and obscure to be useful to anyone, but the Amiga IFF-ILBM viewer supports some of the more esoteric display modes such as HAM6, HAM6 and SHAM, which the DOpus Universal Graphics Viewer does not (properly).
When installing the Recoil viewer, select custom setup and select the Windows thumbnail provider.
More about RECOIL and the viewer (which includes the handler) can be found here: