Let me try this again:
- this post suggests .rar is not supported
- in some previous versions of the app, a bug existed where if the 'best upload' became a .rar file (which I suppose is what you're seeing), then it would show the "RAR is not supported" on every update check (30 minutes)
- .rar support was actually added in 18.5.0
- this fixes the above issue, since the "RAR is not supported" dialog doesn't exist anymore, and the itch app will now simply extract the .rar archive instead of complaining
- please update to the latest of https://itch.io/app
(Also, I'm curious why your itch app didn't auto-update — are you on Linux?)