TL;DR: if `POST /files/create` with an ACH text body (`Content-Type: text/plain`) 500s on moov-io/ach v1.34.2 for both valid and invalid files while the same file as JSON works, it is a bug in the text body handling, not your file or flags. Upgrade to v1.34.4 or later; the fix shipped in PR #1379 and the reporter confirmed it works. No request change needed, just the version bump.

## Steps

1. Upgrade moov-io/ach to v1.34.4+.

2. Re-submit the text file with the bypass flags as before.

   Expected: the file is accepted. Success check: no more 500 on the text path.

## When to use this

- v1.34.2, text/plain 500s, JSON works, flags ignored.

## When NOT to use this

- JSON creation fails too; that is a different problem.
- You are on v1.34.4+ and still see it; report it.

## Compatibility

- moov-io/ach v1.34.4+ (PR #1379).

## Variant phrasings

- "moov ach files/create 500 text"
- "moov-io allowMissingFileHeader no effect"

## Root cause

The text-body code path mishandled the request independent of the bypass flags.

## Edge cases

- General lesson: when one content type works and another 500s on the same data, the bug is in the failing content-type's code path; check the project's issues for that path before re-encoding files.