This should already be possible. Essentially, the way file inputs work is that files within a workflow are passed around by their file names. Whenever you’re processing a file within the workflow, the file is uploaded to the Gumloop system, and the way the file object is fetched between nodes is by referring to it through its file name.
So, if you pass in the file name between the subflows (or nodes in general), it should work directly. There’s no other workaround – you can treat it like any other input.
Below is an example. Let me know if that makes sense and works for you.