Can visitors 'force download' images stored on Amazon S3?
Headers passed back from S3 have to be set either on object upload or copy. You would have to use S3::copyObject() to copy the object to itself and set the appropriate headers (mentioned here: https://stackoverflow.com/a/25933113/2040791)

Alternatively, you can use the method mentioned in the same thread and force a download using HTML5, but that won't work on all browsers.