For some reason the service debugger shows no value for the request.mainpart variable and audits do not display it either.
By default the API Gateway knows of certain Textual Types like text/plain and application/xml. If your client app is using a custom content-type header it may be necessary to let the gateway know that these special definitions should be treated as text. This is done by ensuring your content-type contents (exactly match) a custom entry in Cluster Wide property of contentType.otherTextualTypes
By specifying this cluster wide property you can then override with custom content-types to have the gateway recognize this as a "string" and text..