I am creating a gallery that will automatically display new images from a chosen category and its subcategories. I have set up the categories so that they will apply to media using: register_taxonomy_for_object_type( 'category', 'attachment' );

Now I need to make it so that categories will count the related attachments not just posts.
I found this link How to Override default update_count_callback for category with this code:

function change_category_arg() {
    global $wp_taxonomies;
    if ( ! taxonomy_exists('category') )
        return false;

    $new_arg = &$wp_taxonomies['category']->update_count_callback;
    $new_arg->update_count_callback = 'your_new_arg';

}
add_action( 'init', 'change_category_arg' );

But as of yet I have not figured it out (not sure if it doesn't work or if I'm just not understanding something, such as what would be 'your_new_arg').
I did find the update_count_callback function option when registering a new taxonomy but I don't want to make my own, I want to use it with the existing category taxonomy.

Any help with this is much appreciated. Thanks!