You’ll need to set the following CORS Configuration to your bucket:
*ROCKET_CHAT_DOMAIN* with the domain of your Rocket Chat installation, e.g.
https://chat.example.com. If for some reason you want to make your CORS configuration ridiculously insecure, you can set it to
* to allow CORS requests from any host on the Internet, but do so at your own risk.
This is a companion discussion topic for the original entry at https://rocket.chat/docs/administrator-guides/file-upload/amazon-s3/