public function getMimeBoundary()
    {
        if ($this->mimeBoundary === false) {
            $this->mimeBoundary = null;
            $contentType = $this->getContentType();
            if ($contentType !== null) {
                $this->mimeBoundary = $contentType->getValueFor('boundary');
            }
        }
        return $this->mimeBoundary;
    }