Restful Media-type multipart/form-data incorrectly fills in json object

I'm trying to send a multipart/form-data with the first item being a json object and the second being an image attachment but soapUI automatically fills in the first object as Content-type multipart/form-data. Is there a way to override the content-type setting for the first object?

SoapUI is setting this to:
Content-Type: mulitpart/form-data
Content-Transfer-Encoding: 8bit

But I need the following for the first object:
Content-Disposition: form-data; name="messagePart"
Content-Type: application/json; charset=US-ASCII
Content-Transfer-Encoding: 8bit


I tried to just put that as part of the message but that doesn't override the auto generated items. Any ideas on how I can set this manually?