Need to associate project icon to specific files with a certain extension (e.g., .hypn) for our macOS app developed in Flutter/Dart -- 2

We have a desktop app that works for Windows, macOS, and Linux. The app was developed in Flutter/dart. The proof of concept for the app is completed and working. The app encrypts files for the user. The encrypted files get a special extension (e.g., .hypn). We would like all .hypn files on macOS to be associated with the product icon for the app in macOS. We have this working with no problems on Windows. We are unsure how to do this on macOS. On the Windows side, we added this feature to the Windows installer. We know that it can also be added to the macOS installer as a script to be run during the installation. Although we are open to suggestions. If you have done this sort of project, then please contact us and bid on this project. Thanks.