Add support for upload property

[lasseschou]

The XMLHttpRequestUpload object that is accessible through the upload property is not accessible through the new XHR object. This property, along with the other missing ones, should be surfaced so it doesn't break existing functionality of the original XHR.

[alfishe]

Issue is still actual. Better to add XHR 2 support

[ilinsky]

It is not possible to add any functionality related to file upload with XMLHttpRequest.js library as there is probably no way to provide a fallback implementation for older browsers.

The suggested solution is the following: you include this library only with IE less then 10 (using IE conditional comments), and manage absence of upload functionality there in your code.

<!--[if lte IE 9]>
<script src="XMLHttpRequest.js" type="text/javascript"></script>
<![endif]-->