Filestack (formerly Filepicker) is a service that allows users to store files in a cloud. It has a free plan with some monthly limits: 250 uploads, 500 transformations and 3GB bandwidth. Those numbers get higher if you select one of the paid plans. You can also use external storage (S3, Dropbox…), though you have to select any paid plan to enable this feature. Here’s how to use Filestack in a Rails app.
The filepicker-rails gem makes uploading files to Filestack easy. The files can be selected from one of the very numerous sources, and images can be cropped before upload. Unfortunately, since the gem doesn’t seem to be actively maintained, there’s the risk of encountering some bugs.