XHR multiple image upload

image_upload.js

$(document).ready(function() {

	var fileInput = $('#ImageImage');
	var fileList = $('#file_list');
	var form = $('#ImageAdminAddForm');
	var fileCount = 0;

	// The submit button might have loaded 'disabled' (e.g. if the page was refreshed half-way through an upload) so be sure to remove this.
	$('input[type=submit]', form).removeAttr('disabled');
	
/**
 * Creates a thumbnail preview of the specified file, with progress bar.
 * @param object file The selected file to upload
 * @param int fileNumber	Which number file this is in the batch upload
 */
	function createPreview(file, fileNumber) {
		var li = document.createElement('li'),
			div = document.createElement('div'),
			img,
			progressBarContainer = document.createElement('div'),
			progressBar = document.createElement('div'),
			reader,
			xhr,
			fileInfo;
		
		// If the file is an image and the web browser supports FileReader, present a preview in the file list.
		if(typeof FileReader !== 'undefined' && (/image/i).test(file.type)) {
			img = document.createElement('img');
			img.title = file.name;

			if(window.webkitURL) {
				img.src = window.webkitURL.createObjectURL(file);
			} else {
				img.src = window.URL.createObjectURL(file);
			}

			li.appendChild(img);
		}

		progressBarContainer.className = 'progress_bar_container';
		$(progressBarContainer).attr('id', 'progress_bar_container_' + fileNumber);
		
		progressBar.className = 'progress_bar';
		progressBarContainer.appendChild(progressBar);
		li.appendChild(progressBarContainer);

		$(progressBarContainer).html('Ready to upload…');

		fileList.append(li);
	}

/**
 * Uploads the specified file, alongside all the other fields in the form.
 * @param object file The selected file to upload
 * @param int fileNumber	Which number file this is in the batch upload
 */
	function uploadFile(fileNumber) {
		file = fileInput.prop('files')[ fileNumber ];
	
		console.log('uploading file ' + fileNumber);

		progressBarContainer = $('#progress_bar_container_' + fileNumber);

		progressBar = document.createElement('progress');
		progressBarContainer.html(progressBar);
		
		xhr = new XMLHttpRequest();

		// Update the progress bar as the file uploads.
		xhr.upload.addEventListener('progress', function (evt) {
			if(evt.lengthComputable) {
				percentComplete = (evt.loaded / evt.total) * 100;
				console.log(percentComplete);
				progressBar.max = 100;
				progressBar.value = percentComplete;
			}
		}, false);

		// Show in the progress bar when the upload is complete.
		xhr.addEventListener('load', function () {
			progressBarContainer.addClass('uploaded');
			progressBarContainer.html('<span>&#10003;</span> Done');

			console.log('upload for file ' + fileNumber + ' finished');

			if(fileNumber < fileCount - 1) {
				// As there are more files to upload, call the upload function again.
				uploadFile(++fileNumber);
			} else {
				// As all the files were uploaded, redirect.
				if(typeof xhr.responseText !== 'undefined') {
					window.location = JSON.parse(xhr.responseText).redirect_url;
				} else {
					window.location = document.referrer;
				}
			}
		}, false);

		xhr.open('post', location.pathname + '.json');
		xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');

		// Get the entire contents of the form, including all fields.
		formSubmission = form.clone(true, true);

		// This script aims to upload the images one-by-one, so if multiple images were selected they need to be removed from the submission.
		$('#ImageImage', formSubmission).remove();
				
		formData = new FormData(formSubmission[0]);
		// Now append the one image to be uploaded to the POST request.
		formData.append('data[Image][image]', fileInput.prop('files')[ fileNumber ]);

		xhr.send(formData);
	}

/**
 * Monitors for a new selection of files being made.
 */
	fileInput.change(function() {
		if(typeof this.files !== 'undefined') {
			fileList.empty();
			
			for(var i=0, l=this.files.length; i<l; i++) {
				createPreview(this.files[i], i);
			}
		}
	});

/**
 * Monitors for the form submit button being pressed, and uploads the files.
 */
	form.submit(function(event) {
		fileCount = fileInput.prop('files').length;
		
		if(typeof fileInput.prop('files') !== 'undefined' && fileCount) {
			// Convey that the upload is happening by disabling the submit button and jumping to the first file in the upload list.
			$('input[type=submit]', form).attr('disabled', 'disabled');

			// Scroll up to the file list to show that the upload has begun.
			window.location.hash = 'file_list';
			event.preventDefault();

			uploadFile(0);
		}

		return false;
	});

/**
 * Dynamically updates the selected="selected" attributes on <select> options when a different option is chosen. Otherwise FormData will send values for whatever was selected when the page first loaded, which is not desired.
 */
	$('select', form).change(function() {
		newValue = $(this).val();
		$('option', this).removeAttr('selected');
		$('option[value="' + newValue + '"]', this).attr('selected', 'selected');
	});
});

This helped me wrap my head around this.

SO THANKS! :)