private function getPartHeadersIterator()
    {
        if ($this->part instanceof ParentHeaderPart) {
            return $this->part->getRawHeaderIterator();
        } elseif ($this->part->getParent() !== null && $this->part->getParent()->isMime()) {
            return new ArrayIterator([
                [ 'Content-Type', $this->part->getContentType() ],
                [ 'Content-Disposition', $this->part->getContentDisposition() ],
                [ 'Content-Transfer-Encoding', $this->part->getContentTransferEncoding() ]
            ]);
        }
        return new ArrayIterator();
    }