Start a wiremock server, port 8089 (I was using spring boot annotations but guess the underlying is the same?), I had post in stubFor originally but changed to any(urlMatching...) (see below):
val responseBody = """
Whilst the wiremock server is running, just fire a postman request at:
set the body in postman as form-data (I selected a file with some content)
The use the headers i posted above, and you should see the error?
Hope that helps? Thanks