Add extension mp4

inc/config.php

@@ -754,6 +754,7 @@
 	$config['file_icons']['default'] = 'file.png';
 	$config['file_icons']['zip'] = 'zip.png';
 	$config['file_icons']['webm'] = 'video.png';
+	$config['file_icons']['mp4'] = 'video.png';
 	// Example: Custom thumbnail for certain file extension.
 	// $config['file_icons']['extension'] = 'some_file.png';