upload.js

upload.js

var xhr = _this = this,
        new XMLHttpRequest(),
	fd = new FormData(),
	key = this.email + '/' + epoch.toString() + '/' + file.name;

// Populate the Post paramters.
fd.append('key', key);
fd.append('AWSAccessKeyId', 'XXXXXXXXXXXX');
fd.append('acl', 'private');
fd.append('success_action_redirect', "https://attachments.me/upload_callback")
fd.append('policy', "CnsiZXhwaXJhdGlvbiI6ICIyMDIwLTAxLTAxVDAwOjAwOjAwWiIsCiJjb25kaXRpb25zIjogWyAKICB7ImJ1Y2tldCI6ICJhdHRhY2htZW50c21lLWZpbGUtZHJvcCJ9LCAKICBbInN0YXJ0cy13aXRoIiwgIiRrZXkiLCAiIl0sCiAgeyJhY2wiOiAicHJpdmF0ZSJ9LAogIHsic3VjY2Vzc19hY3Rpb25fcmVkaXJlY3QiOiAiaHR0cHM6Ly9hdHRhY2htZW50cy5tZS91cGxvYWRfY2FsbGJhY2sifSwKICBbInN0YXJ0cy13aXRoIiwgIiRDb250ZW50LVR5cGUiLCAiIl0sCiAgWyJjb250ZW50LWxlbmd0aC1yYW5nZSIsIDAsIDUyNDI4ODAwMF0KXQp9")
fd.append('signature',"IwZctkWS92kelyYv1KfIKqZotwo=")
fd.append('Content-Type', '$Content-Type')
// This file object is retrieved from a file input.
fd.append('file', file);
xhr.open('POST', 'https://attachmentsme-file-drop.s3.amazonaws.com', true);

// Keep track of upload progress so that we can message
// it to the user.
xhr.upload.addEventListener('progress', function(e) {
	if (firstProgressEvent) {
		_this.total += e.total;
	}
	firstProgressEvent = false;
	_this.loaded += (e.loaded - lastBytesLoaded);
	_this.onProgress(_this.loaded / _this.total);
}, false);

// If the upload completes we should decrement the uploads
// currently taking place.
xhr.onreadystatechange = function() {
	if (xhr.readyState != 4)  { return; }
	
	_this.uploadsInProgress --;
	
	// Explicitly trigger our upload complete endpoint. There's a bug with
	// redirects in Chrome which makes this necessary.
	xhr = new XMLHttpRequest();
	xhr.open("GET", "https://attachments.me/upload_callback?key=" + key, false);
	xhr.send(null);
};

http://bencoe.tumblr.com/post/30685403088/browser-side-amazon-s3-uploads-using-cors

If you have the attributes in the form you can use something like this with jQuery.

$('form').on('submit', function(event){
    var data = new FormData(this);
    var xhr = new XMLHttpRequest();
    xhr.upload.addEventListener('progress',function(ev){
        //display progress in some way
    }, false);
    xhr.onreadystatechange = function(ev){
        if(xhr.readyState == 4){
          //complete! - check xhr.status
        }
    };
    xhr.open('POST', , true);
    xhr.send(data);
    return false;
});