It could cost many applications a lot of dev time to switch the adapter and QA everything. I think it would be best to place it in a separate gem and archive it. Then recommend applications begin using ActiveStorage and make the switch when possible.

I'm experimenting with trying to build a generator that installs the adapter in the host application so that we don't have to create an extension. If a user still uses the Paperclip adapter, we can print a deprecation message asking to run the generator to install the adapter. From that moment, they can keep using the adapter, but they will be responsible for maintaining it. I can push some code soon to show better what I mean, but if you have preliminary feedback on the approach, let me know!

@waiting-for-dev @BenMorganIO I opened a PR with the approach proposed, let me know your thoughts! It's not urgent so take your time.