[PROBLEM] Is webp supported for artwork?

[jcucurull]

That would certainly be possible but someone is going to have to verify that it actually works.

Try adding the mime type to /var/www/coverart.php and see what happens.

I did try to add the mime type to the file "coverart-php" but it did not work. The reason is that the mentioned PHP file was not the one used when the thumbnails were created. 

Instead, the file to modify was one called "/var/www/util/thumb-gen.php" that included the same functions copied from the first file. After adding the mime type to the appropriate functions, the albums with "webp" covers were correctly shown (after the corresponding re-generation of the thumbnails).

I include the modified files so Tim can see the differences and integrate them in a new version if he considers it appropriate.

Thanks for the tips @Tim Curtis !