Simply create a /usr/share/thumbnailers/webp.thumbnailer file with the following content:
[Thumbnailer Entry]
Version=1.0
Encoding=UTF-8
Type=X-Thumbnailer
Name=webp Thumbnailer
MimeType=image/webp;
Exec=/usr/bin/convert -thumbnail %s %i %o
Restart tumblerd or log out and back in again for it to take effect.