You may notice that the app pauses file upload when you return to the Home screen or switch to a different app.
That's because the app can only be kept in the background for a small-time (around 1 minute only).
Note: At the moment, you have to keep the app open if you need to upload multiple items or large files. Changing the Auto-Lock timer value to Never in iOS Settings > Display & Brightness should prevent the device from auto-locking, without the need to touch the screen. Once you complete your file upload, do not forget to return to your preferred setting.