Uploading a file with a non-ASCII name (Chinese, emoji, …) failed with
"Failed to execute 'setRequestHeader' on 'XMLHttpRequest': String contains
non ISO-8859-1 code point." The presigned PUT's signed Content-Disposition put
the raw filename in the plain `filename="..."` parameter, which the browser
then rejects when setting it as an XHR request header.
Add a shared attachmentContentDisposition() helper that keeps `filename=`
ASCII-only and carries the real name in `filename*=UTF-8''`, and route all
three construction sites through it so the signed and client-returned values
stay identical (the value is part of the SigV4 signature).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
saltbo