Background upload
Things to consider when backgrounded
iOS
On iOS, a BGContinuedProcessingTask is used to keep the library uploading in the background.
The OS cancels such tasks if system ressources are few. For example, a task will be canceled when opening the camera app.
If the task is cancelled, the upload needs to be resumed by calling the resume() method.
Executing JS code while backgrounded
The progress callback also fires while the app is in the background. The JS side can listen to progress events and react on it.