Add an UploadedFiles param converter

Problem/Motivation

In 📌 [META] Modernise file upload logic Active we have a number of places where we try to get uploaded files from the request.

I think it would also be worth exploring a param converter at the routing level to see if we can upcast the raw POST data into UploadedFiles too, making things even more consistent.

Steps to reproduce

Proposed resolution

Add a param converter to add uploaded files as a parameter to the controller.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet