Error parsing HTTP request using SOAP 1.2 with MTOM (using SOAP UI 5.3)

Resolved: I was on the proverbial 'wild goose chase' Digging deeper, I eventually discovered that the problem was in the WSDL. I was specifying Soap instead of Soap 1.2, which was causing an incorrect request to the server.

In wsld:definitions,

Was using:

xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"

Now using:

xmlns:soap12="http://schemas.xmlsoap.org/wsdl/soap12/"