added support for multiple files echo

Hi,

When I use a form with file input with multiple, the server only echos the last file that was sent instead of echoing a list of the files that were sent.
The reason is that the name of the form value is identical for both files.

I used this API: http://httpbin.org/post
With a simple form and code:

    <form id="myform">
         <input type="file" name="files" multiple>
    </form>
    <button onclick="run()">run</button>
    <script>
    function run() {
        var f = new FormData(document.getElementById("myform"));
        var xhr = new XMLHttpRequest();
        xhr.open("POST", "http://httpbin.org/post", true);
        xhr.send(f);
    }
    </script>

Thanks,
Iftach