I've done 2 Rails apps involving image uploads with paperclip. The first one I did a while ago worked fine locally but I had issues when I deployed to Heroku; I realized I needed to use AWS to enable image uploads.
I did that project a while ago. I recently started another project and tried to incorporate similar functionality. Before I enabled AWS with paperclip when I tried to deploy, I just wanted to test what would happen if I tried to upload an image. To my surprise, it worked without AWS! I was hoping to understand why it didn't work the first time and why it does work now. How does paperclip work with heroku and rails behind the scenes?
It's likely the first application was hosted on the legacy Bamboo stack, that had a read-only file system.
In the new Cedar stack, the file system is no longer read-only, so the upload will not fail. However, you should keep using AWS or any other external storage because Heroku distributes your compiled application across several machines, and it's not guaranteed that the image will be visible from another request. They call it ephemeral filesystem.
In other words, with the Bamboo stack it was easy to understand that you could not store files on the file system, because you were experiencing an immediate crash. With Cedar the upload succeeds, but the uploaded image will be unavailable at anytime in the future.
The moral of the story is: you should keep using AWS or any other storage outside Heroku file-system.
来源：https://stackoverflow.com/questions/20523949/how-to-use-paperclip-with-rails-and-how-does-it-work-in-deployment