By default, attachments are passed into WSIF as DataHandlers. If the part on the message which is the DataHandler maps to a mime:part in the WSDL, then WSIF automatically maps the fully qualified name of the WSDL type to DataHandler.class and sets up that type mapping with the SOAP provider.
In your WSDL, you might have defined a schema for the attachment (for instance as a binary[]). Whether or not you have done this, WSIF silently ignores this mapping and treats the attachment as a DataHandler, unless you have explicitly issued mapType(). WSIF lets the SOAP provider set the MIME content type based on the type of the DataHandler, instead of the mime:content type specified in the WSDL.