Would it be possible to make an extension to allow an app to become the default viewer/editor for a file type?
It cannot be done by any extensions, It should be done with the use of sharing component of another app when the user checks the option "always this file type using the app", which means that it is upto the user whether they want to make it default or not.
Yes it can be implemented.
You just need to add correct intent filter in your app's manifest.