Set thumbnail format to WebP

inc/instance-config.php

@@ -158,6 +158,7 @@ $config['max_images'] = 5;
 $config['image_reject_repost'] = false;
 
 $config['thumb_method'] = 'gm+gifsicle';
+$config['thumb_ext'] = 'webp';
 $config['gnu_md5'] = '1';