Extend `/docx` downloads to `/file` for arbitrary file types

From the PR description

Summary

This extends the existing /docx endpoint to allow for download of arbitrary files, not just DOCX. It consolidates the old /docx endpoint with a new /file endpoint and redirects the former there.

It's just a suggestion to allow other folks to extend it as we have.

Why / Motivation

Folks implementing in-browser editors and document viewers for the miscellaneous supported formats will want access to the raw file bytes as opposed to the results supplied by /display.

Changes

As above.

Tradeoffs & risks

The very minor trade-off is that folks who want to avoid the imperceptible redirect from /docx to /file will need to migrate. Then the /docx endpoint could be deprecated in favor of the more general /file.

How verified

We ran the test suite, which was unaffected by the changes.

Checklist

• Ran the relevant build/test command for the area changed.
• Reviewed git diff and removed unrelated changes.
• Updated docs / env examples if setup, config, or behavior changed.
• No secrets, API keys, real documents, or .env files committed.