INT-1800: Add MTOM Support for WS

JIRA: https://jira.spring.io/browse/INT-1800

The Simple WebService Inbound and Outbound Gateways can operate with `WebServiceMessage`s directly.
This allows to create those messages manually and add attachments to them.
Or, on the other hand, process incoming messages with attachments manually.

For this purpose the `SimpleWebServiceOutboundGateway` is supplied with new `extractPayload` property.
Also the `UnmarshallingTransformer` can now process `MimeMessage` as payload to unmarshal it into object graph with attachments if that

src/reference/asciidoc/whats-new.adoc

@@ -148,4 +148,6 @@ See <<stomp>> for more information.
 
 - The `DefaultSoapHeaderMapper` can now map a `javax.xml.transform.Source` user-defined header to a SOAP header element.
 
+- Simple WebService Inbound and Outbound gateways can now deal with the complete `WebServiceMessage` as a `payload`, allowing the manipulation of MTOM attachments.
+
 See <<ws>> for more information.

src/reference/asciidoc/ws.adoc

@@ -271,3 +271,25 @@ And in the end we have SOAP envelope as:
     </soapenv:Body>
 </soapenv:Envelope>
 ----
+
+[[mtom-support]]
+=== MTOM Support
+
+The Marshalling Inbound and Outbound WebService Gateways support attachments directly via built-in functionality of the marshaller, e.g. `Jaxb2Marshaller` provides the `mtomEnabled` option.
+Starting with _version 5.0_, the Simple WebService Gateways can operate with inbound and outbound `MimeMessage` s directly, which have an API to manipulate attachments.
+When you need to send WebService message with attachments (either a reply from a server, or a client request) you should use the `WebServiceMessageFactory` directly and send a `WebServiceMessage` with attachments as a `payload` to the request or reply channel of the gateway:
+[source, java]
+----
+WebServiceMessageFactory messageFactory = new SaajSoapMessageFactory(MessageFactory.newInstance());
+MimeMessage webServiceMessage = (MimeMessage) messageFactory.createWebServiceMessage();
+
+String request = "<test>foo</test>";
+
+TransformerFactory transformerFactory = TransformerFactory.newInstance();
+Transformer transformer = transformerFactory.newTransformer();
+transformer.transform(new StringSource(request), webServiceMessage.getPayloadResult());
+
+webServiceMessage.addAttachment("myAttachment", new ByteArrayResource("my_data".getBytes()), "plain/text");
+
+this.webServiceChannel.send(new GenericMessage<>(webServiceMessage));
+----

src/reference/asciidoc/xml.adoc

@@ -275,6 +275,10 @@ Custom conversion to a `Source` is also supported by injecting an implementation
 
 NOTE: If a `SourceFactory` is not set explicitly, the property on the `UnmarshallingTransformer` will by default be set to a http://docs.spring.io/spring-integration/api/org/springframework/integration/xml/source/DomSourceFactory.html[DomSourceFactory].
 
+Starting with _version 5.0_, the `UnmarshallingTransformer` also supports an `org.springframework.ws.mime.MimeMessage` as the incoming payload.
+This can be useful in scenarios when we receive a raw `WebServiceMessage` via SOAP with MTOM attachments.
+See <<mtom-support>> for more information.
+
 [source,xml]
 ----
 <bean id="unmarshallingTransformer" class="o.s.i.xml.transformer.UnmarshallingTransformer">