Have image.webp compatible with Thumblist recipe

Hello,

How to make Thumblist recipe friend with webp image ?

In some page, I have 2 images, one is jpg, the other one is webp ;
when running thumblist, to be given the thumbnail of every image, only
the jpg one is printed out.
(:thumblist px=200 quiet=3 usetemplate=1:)

Already added in config is
$ImgExtPattern = "(?:$ImgExtPattern|\\.webp|\\.WEBP)";
with no success.

Thank you,
Gilles.