Once you exit your app, you are pretty much shut down. You can masquerade as a location tracking background agent to remain in the background when you get deactivated, though you'll suck battery and I believe there can only be one of these active at a time. Generally, highly not recommended (and you'll probably fail certification).
A better way to do this if BTS is not to your liking is to use a ResourceIntensiveTask
. This will only be triggered when the user is plugged in and has WiFi but will allow you to run whatever you want for as long as the conditions are met (for example, at night) which should be plenty of time to upload a 3-4 MB file.