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.