Amazon's Cache Control setting has now been added as an option on the putObject method, which lets the browser know the object should be cached when it is accessed.
This is enabled by default on the new version. It is recommended that all users upload as this will lead to cost savings due to less requests being sent to S3.
Normally, even if an item is in the browser cache, it will still check S3 to see if the item is updated, but this tells the browser to just use the cache and not even touch S3.
Thanks to Jerad Sloan for the code.