Does it need to be a multipart for any specific reason? You can also send the file as base64 encoded string in the form attributes/properties.
I am now using a custom Java action to solve the content-type issue with sending form-data files. Please let me know if you want me to share it with you.
Sorry for the late response.
Here is the java action that we are using in the project: https://www.dropbox.com/s/5zem8nzx2y2235a/PostFileMultipartFormdata.mpk?dl=0
Please note that the code might not be perfect. It is not unit tested and code reviewed properly.
As far as I know the Call Rest action does not (yet) support multipart requests. You can only send a FileDocument using Binary for the request.
The RestServices module does support requests where the file is part of a multipart form submit request.
Replying to this old post, because Studio Pro 9.3 has just been released with a new feature: you can now specify headers for the different parts of the form-data request.