Problem
{"ErrorDetail":"Incoming messages must have a body format
of Raw or Xml. Is a ContentTypeMapper set on the
WebHttpBinding?","Status":"INVALID_ARGUMENTS"}
Background
A POST request to a WCF service through Postman (though a request through curl gives the same result).
https://client-webservice.provider.com/WebService/api/rest/
category/getInterestingDetails?ParamID=123456)
Username and password are provided through Basic Auth, which adds another param in addition to the ParamID example parameter.
The service can return JSON. Content-Type is set to application/json.
Solution
Solution in my case (in Postman) was to empty the params beyond the authentication details and place those parameters in the raw request body instead.