>>895
You probably stumbled across some air sellers
cwebp and dwebp are free.

Sorry I'm on Linux so I would just do

find -maxdepth 1 -type f -iname '*.webp' -execdir gm convert {} {}.png \;

http://www.graphicsmagick.org/INSTALL-windows.html#retrieve-install-package
https://sourceforge.net/projects/graphicsmagick/files/graphicsmagick-binaries/1.3.45/

I heard recent Windows versions now have nice and capable terminal as well...