pre_get_posts() - Add CPT to Category and Tag pages

gistfile1.txt

add_filter('pre_get_posts', 'query_post_type');
function query_post_type($query) {
  if(is_category() || is_tag()) {
    $post_type = get_query_var('post_type');
  if($post_type)
	    $post_type = $post_type;
	else
	    $post_type = array('post','press_piece','knowledge_base_entries', 'nav_menu_item');
    $query->set('post_type',$post_type);
	return $query;
    }
}