@moncisss That would be an option in your OS (which you don't mention), not in Opera GX.
In Windows, go to the Windows control panel and select Apps, then Default apps. Then type the extension and choose the correct app. Or right-click on one of the files (on the desktop or in File Manager) and choose Open with, select the correct app, and click on Always. (Sorry, I don't know where those settings are on a Mac,)