If you are retrieving the file from an non-Biztalk web service and sending to another non-Biztalk web service then the easiest way would be to Base64 encode the file stream into an Xml node of the Soap message. Biztalk would not need to decode this data, just send it to the other web service that would decode the base64 back to a file stream.
Alternatively you could use the WCF Adapter and MTOM to move the file payload. This would be more complicated but faster - smaller messages and no base64 encoding/decoding.
