Horde_Mime_Part::setType PHP Method

    public function setType($type)
    {
        /* RFC 2045: Any entity with unrecognized encoding must be treated
         * as if it has a Content-Type of "application/octet-stream"
         * regardless of what the Content-Type field actually says. */
        if (!is_null($this->_transferEncoding)) {
            $this->_headers['content-type']->setContentParamValue($type);
        }
    }

 private function load()
 {
     $headers = [];
     $fetch_query = new \Horde_Imap_Client_Fetch_Query();
     $fetch_query->bodyPart($this->attachmentId);
     $fetch_query->mimeHeader($this->attachmentId);
     $headers = array_merge($headers, ['importance', 'list-post', 'x-priority']);
     $headers[] = 'content-type';
     $fetch_query->headers('imp', $headers, ['cache' => true]);
     // $list is an array of Horde_Imap_Client_Data_Fetch objects.
     $ids = new \Horde_Imap_Client_Ids($this->messageId);
     $headers = $this->conn->fetch($this->mailBox, $fetch_query, ['ids' => $ids]);
     /** @var $fetch Horde_Imap_Client_Data_Fetch */
     if (!isset($headers[$this->messageId])) {
         throw new DoesNotExistException('Unable to load the attachment.');
     }
     $fetch = $headers[$this->messageId];
     $mimeHeaders = $fetch->getMimeHeader($this->attachmentId, Horde_Imap_Client_Data_Fetch::HEADER_PARSE);
     $this->mimePart = new \Horde_Mime_Part();
     // To prevent potential problems with the SOP we serve all files with the
     // MIME type "application/octet-stream"
     $this->mimePart->setType('application/octet-stream');
     // Serve all files with a content-disposition of "attachment" to prevent Cross-Site Scripting
     $this->mimePart->setDisposition('attachment');
     // Extract headers from part
     $contentDisposition = $mimeHeaders->getValue('content-disposition', \Horde_Mime_Headers::VALUE_PARAMS);
     if (!is_null($contentDisposition)) {
         $vars = ['filename'];
         foreach ($contentDisposition as $key => $val) {
             if (in_array($key, $vars)) {
                 $this->mimePart->setDispositionParameter($key, $val);
             }
         }
     } else {
         $contentDisposition = $mimeHeaders->getValue('content-type', \Horde_Mime_Headers::VALUE_PARAMS);
         $vars = ['name'];
         foreach ($contentDisposition as $key => $val) {
             if (in_array($key, $vars)) {
                 $this->mimePart->setContentTypeParameter($key, $val);
             }
         }
     }
     /* Content transfer encoding. */
     if ($tmp = $mimeHeaders->getValue('content-transfer-encoding')) {
         $this->mimePart->setTransferEncoding($tmp);
     }
     $body = $fetch->getBodyPart($this->attachmentId);
     $this->mimePart->setContents($body);
 }