I don't think you can do this. Any non-privleged app could implement the view activity for this mime type and get access to the unencrypted media, defeating the DRM.

The previous solution worked because the "view" activity was not really a normal activity but was invoked directly by gecko when the user clicked on an oma.drm link

In order to keep the content locked, the download manager should just stay out of the way and let the FL app do everything, or we should merge the FL app into the download manager app (which I don't want to do) or we need a private or certified-only communication channel between the download manager and the FL app.