Multipart/form-data with JSON

Hi,
im my custom app, I am trying to submit the following request, but it seems to be impossible:

POST <BASE-URL>/documents
Content-Type: multipart/form-data; boundary=<BOUNDARY> 
Transfer-Encoding: chunked
--<BOUNDARY>
Content-Disposition: form-data; name=metadata 
Content-Type: application/json
{
  "metadata":  {
   "document" :  {
        "filename" : <filename>,
        "title" : <title>
     }
  }
}
--<BOUNDARY>
Content-Disposition: form-data; name=document; filename=<filename> 
Content-Type: <mime-type>
<File contents> 
--<BOUNDARY>--

The API documentation says:

Note that both body parts are required and that their order is fixed: The “metadata” body part comes first, and the “document” body part comes second. Upload requests missing a part, or that contain parts in the wrong order, will not succeed.

I tried this request:

{
	"url": "/documents",
	"method": "POST",
	"type": "multipart/form-data",
	"headers": {
		"Content-Type": "multipart/form-data"
	},
	"qs": { },
	"body": {
        "metadata": {
            "value": "{}",
            "options": {
				"type": "application/json"
            }
        },
        "document": {
            "value": "{{parameters.file}}",
            "options": {
                "filename": "{{parameters.filename}}"
            }
        }
	},
	"response": {
		"output": "{{body}}"
	}
}

but the server error message is: Bodypart metadata is not of type application/json

Is it possible to set the content type for the json part?

Best Regards
Jörg