WordPress: Custom class and rel to attachments

<?php
//add custom class to attachment image
add_filter('get_image_tag_class','add_image_class');
function add_image_class($class){
    $class .= ' attachment';
    return $class;
}

//add rel attribute to attachment link
add_filter('wp_get_attachment_link', 'add_rel_to_attachments');
function add_rel_to_attachments($link) {
    global $post;
    //check if rel already exists
    if (strpos($link, ' rel="') == false)
        return str_replace('<a href', '<a rel="lightbox" href', $link);
    else
        return $link;
}
?>