I use Endeavour OS (Arch based btw) and I’ve noticed that when I select to choose a new image as my wallpaper, it doesn’t support the .webp format.
Does plasma desktop not support .webp images as wallpaper? Am I doing something wrong?
Check if you have qt5-imageformats package installed. It is an optional dependency for kio-extras package that is not installed by default on Arch distros.
I am pretty sure webp works with the wallpaper picker on plasma5 from my experience.