It looks like future versions of Rails (> 5.1) will ship with Active Storage. In their own words:
Active Storage makes it simple to upload and reference files in cloud services, like Amazon S3 or Google Cloud Storage, and attach those files to Active Records. It also provides a disk service for testing or local deployments, but the focus is on cloud storage.
This first-party solution should prove to be handy for new Rails projects. Especially in their infancy.
I look forward to doing a bit more research and seeing what the integration between S3 and Active Storage looks like.