protected function file_mod($file)
    {
        $fs  = false;
        $ext = substr($file, strrpos($file, '.') + 1);

        // use minified file if exists (not in development mode)
        if (!$this->devel_mode && !preg_match('/\.min\.' . $ext . '$/', $file)) {
            $minified_file = substr($file, 0, strlen($ext) * -1) . 'min.' . $ext;
            if ($fs = @filemtime($this->assets_dir . $minified_file)) {
                return $minified_file . '?s=' . $fs;
            }
        }

        if ($fs = @filemtime($this->assets_dir . $file)) {
            $file .= '?s=' . $fs;
        }

        return $file;
    }