Send flat file to SOAP Receive Adapter through Web Services

I have published the orchestration as a Web Service for user to upload XML file or Flat file and then feed the data into the database. As of XML file, I have created the message type as an untyped XMLDocument and filtered into the Receive Shape of the Orchestration, and everything works fine. However, I have got the problem with Flat File. I tried to use System.String or System.Byte[] as the message type in the published Web Service, and applied a Flat File receive pipeline with the corresponding Flat File Schema after the processing of SOAP receive adapter, and the pipe line always returned an error something like “You need to assign the root reference property in your Flat File Schema”. Of course I have assigned that property. I get confused and really appreciate someone who can give me some hints about this flat file problem.

Best Regards

William