Extend the list of MIME-types supported by the builtin uploader in 3.3

I’ve used this to add webm support to the wordpress 3.3.1 uploader.

add_filter('upload_mimes', array(&$this, 'addUploadMimes'), 1, 1);

public function addUploadMimes($mimes){
         $mimes = array_merge($mimes, array(
                'webm' => 'video/webm'
             ));
            return $mimes;
    }

I had to make it run with a high priority to stick.