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.
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.
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.
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.