A parameter or return value may be represented as an attachment.
In such cases, the ParameterDescription or OperationDescription has
an AttachmentDescription defining the attachment information.
Note that an Attachment may be one of 3 types: SWA, SWAREF and MTOM.
SWAREF and MTOM attachments have an xml element (either in the body or header)
that references the attachment part.
SWA attachments are represented as an attachment part. There are is no
xml element that references the attachment.