How to send multipart/form-data to OSB 12C proxy service?

I am writing an OSB service for inbound operations where a client is calling my proxy service which then uses the REST adapter to call an API on my internal system.

The external system is sending an HTTP POST call with parameters encoded as name/value pairs; e.g. parm1=abc&parm2=def

The problem I am facing is that the call to my proxy service comes in with the Content-Type set as multipart/form-data while OSB only works with application/x-www-form-urlencoded

I tried setting the payload type, but that generates an error that the media type is not supported.