Javascript XHR send multipart/form-data

I'm trying to send a multipart/form-data content-type request:

var xhr = new XMLHttpRequest();

xhr.onreadystatechange = function(){
    if(xhr.readyState==4){
         alert(xhr.responseText);
    }
}

xhr.open("POST", url, true);
xhr.setRequestHeader("Content-Type","multipart/form-data; boundary=---------------------------275932272513031");

xhr.send('-----------------------------275932272513031 Content-Disposition: form-data; name="name"

test

----------------------------275932272513031--');

Then in php I just print the $_POST array

print_r($_POST);

But I get an empty array each time. I expect to see

Array (
    name => "test"
)

What am I doing wrong?