protected function becomeMimePart()
    {
        $part = new parent($this->getHeaders()->newInstance(), $this->getEncoder(),
            $this->getCache(), $this->getIdGenerator(), $this->userCharset
            );
        $part->setContentType($this->userContentType);
        $part->setBody($this->getBody());
        $part->setFormat($this->userFormat);
        $part->setDelSp($this->userDelSp);
        $part->setNestingLevel($this->getTopNestingLevel());

        return $part;
    }