While XML-based message processing has many advantages, at times you may need to send data as an attachment to a SOAP message rather than embedding it directly into the XML document. This is desirable when the data is of a binary format, such as images, WAV files, PDF documents, and even a separate XML document.

The W3C supports this functionality through its SOAP Messages with Attachments (SwA) specification. This specification describes how to embed a SOAP message inside of a Multipurpose Internet Mail Extensions (MIME) document so that the message processing is not changed.

In WSDL, a MIME attachment's format is defined concretely by the binding, while its datatype is defined in the WSDL types element or in a referenced or embedded XML Schema.