PHP session.upload_progress没有返回任何值

I want to make use of upload_progress session introduced in PHP 5.4. But I can't get any result.

Here is my form:

<form enctype="formdata/multipart">
<input type="hidden" id='prog' value='test' name="<?php echo ini_get("session.upload_progress.name"); ?>">
    <label>Upload Your Files</label><br />
    <input name="file" type="file" id="file"><br />
    <input name="send" value="Upload!" id="uploadButton" type="submit" />

    <br />

</form>

And here is my Javascript:

sub.addEventListener("click", function(event){

            event.preventDefault();
            event.stopPropagation();

            formData.append("file", file.files[0]);

            var http = new XMLHttpRequest();
            http.open("POST", "upload.php");

            http.setRequestHeader("Cache-Control", "no-cache");

            http.addEventListener("load", function(){
                console.log("Finished.");
            }, false);


            var myprogfield = document.getElementById("prog");
            formData.append(myprogfield.getAttribute("name"), myprogfield.value);

            http.send(formData);



            setInterval(function(){
                var http = new XMLHttpRequest();
                http.open("GET", "progress.php");               
                http.send(null);
            }, 500);


        }, false);

As you can see, I use the following to send a request to progress.php to get a result from it.

    setInterval(function(){
        var http = new XMLHttpRequest();
        http.open("GET", "progress.php");               
        http.send(null);
    }, 500);

Here is progress.php, I echo() the whole $_SESSION each time to get all values of $_SESSION, but each time it sends it null.

<?php
session_start();

var_dump($_SESSION);