public function addFile($file)
    {
        if (is_string($file)) {
            $file = array($file);
        }

        if (is_array($file)) {
            foreach ($file as $name) {
                if (!isset($this->_files[$name]) && !empty($name)) {
                    $this->_files[$name] = $name;
                }
            }
        }

        return $this;
    }