Hello,
It is maybe late, but will help some users.
In addition to "ps8modules.com" answer, you also need to change the function isCorrectImageFileExt on the "./classes/ImageManager.php" file:
After this code:
// Filter on file extension
if ($authorizedExtensions === null) {
$authorizedExtensions = ['gif', 'jpg', 'jpeg', 'jpe', 'png', 'webp'];
}
Add this:
if(!in_array('webp', $authorizedExtensions)){
$authorizedExtensions[] = 'webp';
}
Good luck