Add "wrapper" to make the purpose of different fileInput functions more obvious

## Context

Part of action items for [2026-09-02-file-attachments-not-included-in-form-submission](https://docs.google.com/document/d/1_ZQdO6sa2IobYK4gaEptT2k45gUJHU7mh4nj5vK77QI/edit?tab=t.0)

When saving and resuming a form with files previously attached, we had a regression that caused an incident affecting end users.

**To help avoid this in the future, we might consider:**

- Creating a wrapper or equivalent abstraction that makes intentional nulling of file input values explicit for relevant actions – labeling / naming things in code
- If downloading html or uploading content = same function, more obvious to dev using the function

No files, tests, or entry points are named in the issue. First locate the different fileInput functions involved in saving, resuming, downloading HTML, and uploading content; done means their intended behavior, especially intentional nulling, is explicit through a wrapper or equivalent abstraction.

Written by the indexing model from the issue text.