In Mendix 8, the HttpRequest/Content is left empty when the Content-Type is multipart/form-data.
You can handle these type of requests in two ways:
If you set the log to trace what does the logs tell you? Because to me this looks like the other party is at fault here. I can not think of any reason why something on the Mendix end would strip the content. But the logs might tell you something here.
Why are you using form-data instead of json via raw json or export mapping?
I had the same issue in the past. You need to set your headers (Content-Type and Accept) properly. Also define your body in a correct way, with a correct match to your headers. E.g. if your content-type is application/x-www-form-urlencoded, then you need to set the custom template with an & separated key/value string.