add a webm icon

inc/config.php

@@ -683,6 +683,7 @@
 	// Thumbnail to use for the non-image file uploads.
 	$config['file_icons']['default'] = 'file.png';
 	$config['file_icons']['zip'] = 'zip.png';
+	$config['file_icons']['webm'] = 'video.png';
 	// Example: Custom thumbnail for certain file extension.
 	// $config['file_icons']['extension'] = 'some_file.png';