What cloud providers are supported? This addon is compatible with any OpenStack Swift API or S3-compatible provider. The ones specifically tested are OVH Object Storage (uses OpenStack Swift API) and Digital Ocean Spaces (reflink) (S3 API).
How can I move my attachments to cloud storage or back? I recommend using rclone. See documentation for OpenStack Swift and S3. You can (and should) also set up backups for your object storage container, rclone can help you do that.
Not that this addon does NOT provide any way to migrate your existing attachments to cloud storage (or back). You should be able to do that yourself.
How was this addon tested? This addon was in continuous use ona forum with ~1000 daily active registered users for many years. First implementation of this addon worked on XF 1.5. Obviously, this is a rewrite for XF 2.2 and is a bit simpler given XenForo's use of Flysystem. The extra features beyond simply plugging in a storage adapter as outlined here are a product of further attempt at optimizing a community with a significant number of attachments.
How to report bugs? This is a first version public version of this addon. Bugs are expected. Feel free to report them in the discussion thread.
Can I use it on a commercial site/forum?
No, this license is currently distributed for free under a Creative Commons Attribution-NonCommercial 4.0 International license. You cannot use or redistribute it in any commercial way.
If you are interested in a commercial license, let me know via PM.
Do you offer support? This addon comes with "community support" and is released free in hopes that it is useful to you. You can ask questions and seek help here and I'll try to answer but it will be done on a best effort basis.