Saving WP Media Gallery custom fields

function save_custom_media_gallery_fields($post, $attachment) {
    if( isset($attachment['custom_link']) ){
        update_post_meta($post['ID'], '_custom_link', $attachment['custom_link']);
    }
    return $post;
}
add_filter("attachment_fields_to_save", "save_custom_media_gallery_fields", null , 2);