Alternative Download With Amazon S3

You could require all your users to use a browser that supports
downloading in the background ... or use some kind of waiting or
pending animation while the file is being retrieved. You know, like
those pages that use a meta refresh tag and show a message like
"Please wait while your file is being retrieved, if you don't see the
download screen, click here (with some link that just re-executes the
same link".

Does that work for you?

I think this rails' fault: the response won't get out to the user's web browser until rails has finished, whereas what you want to do is to stream it. Not sure how you get round this.