Validator: added WebP to error message

Author: dg

src/Forms/Validator.php

@@ -41,7 +41,7 @@ class Validator
 		Form::MAX_FILE_SIZE => 'The size of the uploaded file can be up to %d bytes.',
 		Form::MAX_POST_SIZE => 'The uploaded data exceeds the limit of %d bytes.',
 		Form::MIME_TYPE => 'The uploaded file is not in the expected format.',
-		Form::IMAGE => 'The uploaded file must be image in format JPEG, GIF or PNG.',
+		Form::IMAGE => 'The uploaded file must be image in format JPEG, GIF, PNG or WebP.',
 		Controls\SelectBox::VALID => 'Please select a valid option.',
 		Controls\UploadControl::VALID => 'An error occurred during file upload.',
 	];