Content-Type for SOAP with Attachments (MTOM)

MTOM / multipart/related

MTOM (Message Transmission Optimization Mechanism) uses multipart/related to send SOAP messages with binary attachments efficiently, avoiding Base64 encoding overhead.

The Header Value

Content-Type: multipart/related; type="application/xop+xml"; start="<root.message@example.com>"; start-info="application/soap+xml"; boundary=MIME_boundary

Parameters Explained

Parameter	Purpose
type	MIME type of the root part (application/xop+xml for MTOM)
start	Content-ID of the root part
start-info	Original Content-Type of the root (before XOP processing)
boundary	Boundary string separating parts

Message Structure

--MIME_boundary
Content-Type: application/xop+xml; charset=utf-8; type="application/soap+xml"
Content-ID: <root.message@example.com>
Content-Transfer-Encoding: 8bit

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
  <soap:Body>
    <UploadDocument>
      <data>
        <xop:Include href="cid:attachment1@example.com"
                     xmlns:xop="http://www.w3.org/2004/08/xop/include"/>
      </data>
    </UploadDocument>
  </soap:Body>
</soap:Envelope>

--MIME_boundary
Content-Type: application/pdf
Content-ID: <attachment1@example.com>
Content-Transfer-Encoding: binary

[Raw PDF binary data - NOT Base64 encoded]

--MIME_boundary--

Why MTOM?

Without MTOM, binary data in SOAP must be Base64-encoded, increasing size by ~33%. MTOM uses XOP (XML-binary Optimized Packaging) to reference binary parts directly, keeping binary data in its raw form.

When to Use

• Large file attachments in SOAP services
• High-performance enterprise integrations
• Medical imaging (DICOM over SOAP)
• Document management systems