Gets or sets ID to bind the Upload object with the upload process and progress bar.
The ID of the Upload object instanse.
You must set the upload ID in order to track an upload progress. Upload ID must to be unique. If an upload with such ID is already present, new upload rejected (save method fails).
In a case the upload ID hasn’t been set it’s generated in the save method. You can’t find it out until the upload completes.
To get know an upload progress a user creates a special page with a Progress object. This object queries about how much bytes have read, how many time left and so on. But the Progress object must be aware the upload it is asked for. It is a place where upload ID comes again. User must set ID property of the Progress object to get data needed.
Usually a user creates unique ID and sends it both to upload and progress pages by GET method (at URL).