WordPress: Only load css on custom post type edit page

// stylesheet used by all similar meta boxes
add_action('admin_init','mb_enqueue');
function mb_enqueue() {
  // Get the globals that tell us where we are in the admin.
  global $pagenow, $typenow;
  // Sometimes $typenow is not available, so let's check and get it if needed.
  if (empty($typenow) && !empty($_GET['post'])) {
    $post = get_post($_GET['post']);
    $typenow = $post->post_type;
  }
  // Only show our scripts on the admin pages they are used on to prevent possible conflicts with other scripts.
  if (($pagenow == 'post.php' || $pagenow == 'post-new.php') && $typenow == 'portfolios') {
    // WP Alchemy Stylesheet
    wp_enqueue_style('custom_meta_css', _TEMPLATEURL . '/lib/admin/metaboxes/mb_style.css');
    // WP Alchemy related JS
    wp_register_script('my-custom_meta_js',  _TEMPLATEURL . '/lib/admin/metaboxes/mb_global.js', array('jquery','media-upload','thickbox'));
    wp_enqueue_script('my-custom_meta_js');
  }
}