WordPress - Display custom post types on the same category page as your default posts

wordpress-add-categories-to-a-custom-post-type

add_filter('pre_get_posts', 'query_post_type');
function query_post_type($query) {
  if( is_category() ) {
    $post_type = get_query_var('post_type');
    if($post_type)
        $post_type = $post_type;
    else
        $post_type = array('nav_menu_item', 'post', 'your-custom-posttype'); // don't forget nav_menu_item to allow menus to work!
    $query->set('post_type',$post_type);
    return $query;
    }
}