file_storage: S3 presigned downloads lack Content-Disposition, so files save under their storage key #489

modules/file_storage/file_storage/backends/s3.py presigned_get_url() calls generate_presigned_url("get_object", Params={"Bucket", "Key"}) with no ResponseContentDisposition (or ResponseContentType). The streamed path in serving.py sets those headers, but on a presigning backend the browser is redirected to S3 and saves the file under its opaque storage key instead of its original filename.

Fix direction: pass ResponseContentDisposition (RFC 6266/5987-encoded original filename, attachment vs inline as the streamed path decides) and ResponseContentType through presigned_get_url; add a test asserting the presigned URL carries both.

Found during the multipart upload work (#454, #483).

https://claude.ai/code/session_01M9neheZZEe3sVpDi2S3zT4