How to pass a file with node POST (using with a RESTful API)

I was hopeing some RESTful node (in particular POST) experts can help me out here:

When using Postman to execute a POST to an API, everything works fine -

I am using a token for authentication in the header, a json string in the body. Also in the body, I upload a file (which is then used by the system I access). Postman offers a file-select dialogbox, there is not "string" one can view to see how it translates (at least I can't find anything of that sort).

Now if I want to do this within Knime I am not sure how to pass the file.

A test passing a JSON string for a case where I don't require a file works fine. But for cases when I want to refer to a file - I am not sure how.

In Postman it looks like there are two body portions, whereas the Knime node only has one body portion.

Any suggestions? 

(PS: I am not an expert in RESTful or JSON, I usually "make it work" though.)