When you double-click a file on your Mac, the operating system will automatically open the file using the program assigned to that type of file. It is possible, though, to open the file using ...

Click Open. If the app that you want to use to open the file type is greyed out, it's because macOS doesn't think there's a valid association between the two.

When you open files that have a particular file extension, Windows will look at its configuration to determine what the default program should be in order to work with these files. The default ...