POSTing a file to a https REST service

I need to POST a file to a https REST service. I want to confirm the following.

1. With the default 'Call REST service' i'm not able to POST the file with Content-Type: multipart. As fas as I can see I can only wrap something in XML or JSON and than sent it. I assume that the 'Call REST service' action can use the certificates uploaded in the cloud portal, just as you can with SOAP webservice calls.
2. With the REST module from the App Store you can POST the contents of FileDocuments (or subclasses) as multipart, but that module doesn't seem to be able to use the certificate out of the box.

So my current understanding is that I need a custom solution and I can't use any platform or App Store functionality.