onCancel Callback

Detect when a user cancels an upload task.

(function() {
 var logEl = document.getElementById('log');
 function log(m) { var d = document.createElement('div'); d.className='pkg-log-entry'; d.textContent=m; logEl.appendChild(d); logEl.scrollTop=logEl.scrollHeight; }
 new MultipleUpload('#demo', {
 uploadUrl: '/api/upload',
 multiple: true,
 onCancel: function(file, task) {
 log('Cancelled: ' + (file ? file.name : task.fileName));
 },
 onTaskStart: function(t) { log('Started: ' + t.fileName); }
 });
})();

Knowing when the user backs out

onCancel fires when an upload is cancelled, which is your cue to undo whatever you set up when it started: re-enable a submit button, clear a placeholder row, release a reserved slot. Pairing it with onTaskStart keeps that bookkeeping symmetrical.

Cancelling the request is not cleaning up

Aborting the transfer stops the browser sending; it does not remove whatever the server already wrote. With plain uploads that can leave a partial file, so either write to a staging location that a later step promotes, or have the cancel path call your delete endpoint. Chunked uploads sidestep it: an incomplete chunk set was never assembled.