public function getContentTransferEncoding($default = '7bit')
    {
        static $translated = [
            'x-uue' => 'x-uuencode',
            'uue' => 'x-uuencode',
            'uuencode' => 'x-uuencode'
        ];
        $type = strtolower($this->getHeaderValue('Content-Transfer-Encoding', $default));
        if (isset($translated[$type])) {
            return $translated[$type];
        }
        return $type;
    }