You do realize that you can't update the tile constantly from a PeriodicTask right? The minimum refresh time is 30 minutes, unless you do push notifications (which I here, assume you won't, since it's a client-side operation).
And the tile itself doesn't make much sense. And I don't think using a task that can maximum run for 25 seconds makes a aweful lot of sense either for uploading a file.
So I guess we can conclude that it'll not be possible to do this. Instead , you should allow the application to run in the background, by setting the ApplicationIdleDetectionMode
to Disabled
PhoneApplicationService.Current.ApplicationIdleDetectionMode = IdleDetecetionMode.Diabled"
Then the user can press the Home button, and the download will progress in the background.