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
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