The API allows you to launch the user’s default app for a file type or protocol. See UnityEngine.WSA.Launcher for more information.
You can also enable your app to be the default app for a file type or protocol. You can set this information in Player settings. More settings are available in Package.appxmanifest -> Declarations.
You can download an example Project here.
You can read more here - Assocation launching.
• 2017–05–16 Page amended