In the client classes of the external  webservice, i  changed for the "DocumentData" property, in several places, ,"%xsd.base64Binary" to "%Stream.GlobalCharacter". I recompiled the classes.

After this, in the "Ensemble" connector, i had to do the conversion to Base64 imyself.

Indeed, with "%xsd.base64Binary" the conversion to base64 was done automatically during the transfer.

Now, in my example, fileStream is a base64 stream, and here's how my "DocumentData" property is assigned:

do tsRequest.DocumentData.CopyFrom (fileStream)

I tested and the return from the remote webservice is correct with the same hashing values.

Thank you very much for your help.