Are you perceiving this problem is new with Metro? Conceptually it seems like kind of an old issue to me...
Is file extension managment something that's been advertised as updated in the brave new Metro world? I must have missed it if so.
If not, you may be thinking about this in slightly the wrong way... Your app is not in charge - the user is.
I can think of two scenarios:
1. Another app that does something unrelated to what your app does
happens to come after you and associate itself with the same extension. It's possible you chose the extension badly
in this case, or are just unlucky. Your recourse might be to have the software check for associations being taken away from your app, either when your app is run or maybe in a service or applet that runs in the background. You
should still let the user know about the problem and have the final decision what to do about it. But beware, users sometimes hate being bothered. Longer term, you would want to consider using a different file extension for
your new releases.
2. Another app that does something similar to what your app does overwrites the file extension associations. This is the kind of thing you might see with .jpg for example. In that case it seems entirely possible
that the user has decided to use a different app than yours to handle those files by default. It is up to the user what app he/she wants associated with a particular file extension. If you locked them out of doing that,
you'd be hated in a hurry. In this case your recourse would simply be to make the best app for the job, so every user would install yours and not the competition's.
-Noel
