We make use of Filestack for uploading files to S3 for some of our internal tooling. It's pretty plug-and-play. The developer documentation is pretty decent and certain feature requirements (controlling for upload size) are easy to apply. Review collected by and hosted on G2.com.
Ideally, Filestack would provide some better solution for managing multiple types of environments. Predecessors to my team had things configured so that one of our qa environments used the production account, because the non-production Filestack account was non-paid and thus did not have the capability to upload to S3. It took us a decent amount of time to track this down.
The admin interface is also a bit clunky in some regards. There's something that seems slightly forced about requiring selection of a plan/app for certain activities, although to be fair it's been some time since I've needed to go in there. Review collected by and hosted on G2.com.
