I am still missing something. Since

config.Formatters.XmlFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/octet-stream"));

didn't work I have more debug options by setting a custom configuration like:

            GlobalConfiguration.Configuration.Formatters.Add(new StreamFormatter());

But the method OnReadFromStream in the custom formatter is never called (I attached the VS debugger to the w3wp process). What am I missing?

NOTE: For some reason adding a new formatter works now. But adding application/octet-stream to XmlFormatter does not work. At least I have something that works even if I don't understand why adding the type to existing XmlFormatter doesn't.