I am trying to port across a Workflow from U7 to U13, but the docs are rather lacking still, what method needs to be used to pull out the form attachment?

If you iterate over all the

RecordFields

on a record-aka-submission, surely one of them is the uploaded file?

A fair point but I am wondering how to get at it, since the file originally was pulled using IMediaFileSystem, and Current.MediaFileSystem which doesn't exist anymore but I can't find any docs suggesting the preferred new approach

Can you verify the upload is stored as a media item in the media bank? What does the reference to the upload aka media item look like?