Existem algumas maneiras de você fazer isso para solucionar essa limitação que escreveu sobre onde ele não permite que você selecione outros aplicativos para associar etc., mas vou escrever uma solução simples que você possa usar.
Create a dummy file with the extension you wish to associate with a specific program if you don't have an actual file already (e.g.
test.region) right-click on it and select Open With
When the How do you want to open this file? window pops up, ensure that the option Always use this app to open <
.ext> files is checked and then select More apps
You will then have a scroll bar that you can navigate to select the app you want the file to open with if it's listed. Otherwise, at the bottom of the scrollable list you can select the option Look for another app on this PC. Then when the Open with window opens you can navigate the file system and find the app you want the file to open with and double click it and you'll be all set.
