Hi Jan Joost,
It seems that the message is send using MTOM.
Instead of providing the binary content as Base64-encoded characters - contained as element text in the SOAP message - a multipart mime format is used, to devide the binary content from the SOAP message. Means: the SOAP message itself is contained in one part, and the binary content is in a separate part. The content element in the SOAP message just has a reference to the binary part.
For more information about MTOM please take a look at the following website: https://dzone.com/articles/use-mtom-to-efficiently-transmit-binary-content-in.