add_filter( 'query_vars', 'add_query_vars');
function add_query_vars($vars){
    $vars[] = "photos";
    return $vars;
}
function photos_add_single-photos_endpoint() {
    add_rewrite_endpoint( 'photos', EP_PERMALINK );
}
add_action( 'init', 'photos_add_single-photos_endpoint' );
function project_attachments_template( $single_template ){
    global $wp_query;
    if ( isset( $wp_query->query_vars['photos'] ) ) {
        $single_template = get_stylesheet_directory() . '/single-photos.php';
    }
    return $single_template;
}
add_filter( 'single_template', 'project_attachments_template' );