OK... thought I would try this myself since I am living this problem even 4 years down the road with the latest TC version.
As proposed here, modifying the wincmd.ini at the [Lister] position did not work... I tried a few mods to no avail. And then it hit me, why do we stick the mod down there if we are trying to modify IrfanView (since that is what TC is calling for).
So instead, I re-opened a fresh version of wincmd.ini (I had saved a copy before modifying anything), and went around the Irfanview area of the wincmd.ini file and added the line thus :
And it worked right there and then. Verified it was not just the webp file I was testing that was running true, I accessed a full folder of webp files on my drive and all of them had a static preview thumbnail appear in TC, and each file became viewable in Lister as a static picture (the files are all active, gif style, files).
I am not a programmer, and I know that the position isn't always critical within a *.ini file, but it seems that in this case, it is! Hope this helps others! Best Regards....
P.S. Make sure you make the appropriate modifications in the IrfanView settings (you need a plugin in IrfanView for it to read "webby" webp and webm files, before doing the upper mentioned modification in TC for it to fetch IrfanView when required).