File attachments (for non-API users)

### Description:

All **clients who need to receive documents as part of form submissions can incorporate file upload in their forms and retrieve files, without requiring development support,** as evidenced by little to no support tickets pertaining to file upload feature availability requests.

## Context

https://gcdigital.slack.com/archives/C09CM76GCS0/p1786369044020289

> It would be helpful if GC Forms could incorporate functionality to allow files to be uploaded, without needing to set up API retrieval. This would be useful for routine tasks where setting up an API is overly complex (e.g. completing a survey for meeting registration where attendees need to provide documents).

The issue names no files, tests, or entry points. First clarify the upload, storage, access, and retrieval scope for non-API users, along with security and acceptance criteria. Done means clients can include file uploads in forms and retrieve submitted files without development support or API setup.

Written by the indexing model from the issue text.