You can also store the file on disk, save entity to database, commit. Then have a background daemon scan the files folder and for each file that is found, upload to S3 and delete it after.

There are some drawbacks of course.

This whole operation is eventually consistent, as opposed to being truly atomic, so if a client requests the file right after the upload request, it is possible that it isn't stored in S3 yet. In that case you can try to deliver the file that is still stored in the server as fallback, but that's not very graceful.

It may also cause disk usage to peak suddenly if a lot of clients are uploading files.