Hmm not sure if this is going to work but it might be worth a try.
Download the wsdl and change it by adding an extra field to the response. Make it minOccurs=0. If you import this wsdl it should change your response to a complex response making you able to map your response and in a mapping you are able to set a binary field to the content field of a filedocument.
Just modifying the wsdl with the bogus line was the answer:
<s:element name="GetAttachmentResponse">
<s:complexType>
<s:sequence>
<s:element minOccurs="0" maxOccurs="1" name="GetAttachmentResult" type="s:base64Binary" />
<s:element minOccurs="0" maxOccurs="1" name="Bogus" type="s:string" />
</s:sequence>
</s:complexType>