protected function fetchAttachment($structure, $partNumber) {

        $oAttachment = new Attachment($this, $structure, $partNumber);

        if ($oAttachment->getName() !== null) {
            if ($oAttachment->getId() !== null) {
                $this->attachments->put($oAttachment->getId(), $oAttachment);
            } else {
                $this->attachments->push($oAttachment);
            }
        }
    }