How to attach a file when using multipart/mixed using

I also am attempting something very similar to this. I need to send a multipart/mixed POST with two parts: binary object meta-data (content-type application/json), followed by the binary object data itself (content-type dependent upon the file: image, document, whatever).

There seems another similar post on this topic, with no response. Does this mean that SoapUI can't do this?

Thanks.