By associating your app with a file extension, the Windows Phone OS sends requests to open any files of the registered type to your app. It does so by launching your app and providing a deep link URI that contains a file token—a guid that is used to retrieve a file from storage.
It is your app’s responsibility to recognize the incoming file launch URI and redirect to a page that can handle the request.
When an app is the subject of a file association, it is launched and navigated to a URI beginning with this text: /FileTypeAssociation.
The value of a fileToken query string parameter is used to retrieve the file from storage. The following example shows a deep-link URI from a file launch: ...