protected function contentInfoString($type = null, $file = null, $filename = null, $charset = null)
    {
        $data = $this->contentInfoArray($type, $file, $filename, $charset);
        $fields = array();

        foreach ($data as $key => $value) {
            $fields[] = EmailHelper::fold(sprintf('%s: %s', $key, $value));
        }

        return !empty($fields) ? implode("\r\n", $fields)."\r\n\r\n" : null;
    }