public function getContentStream($charset = MailMimeParser::DEFAULT_CHARSET)
    {
        if ($this->hasContent()) {
            $tr = ($this->ignoreTransferEncoding) ? '' : $this->getContentTransferEncoding();
            $ch = ($this->charsetOverride !== null) ? $this->charsetOverride : $this->getCharset();
            return $this->partStreamFilterManager->getContentStream(
                $tr,
                $ch,
                $charset
            );
        }
        return null;
    }