empty responseText from xmlhttprequest

I'm upload an file using ajax,why the responseText from xmlhttprequest.responseText is returned empty?

My code:

req = new XMLHttpRequest(); 
req.file = file; 
req.addEventListener('change', changeProgress); 
 req.onreadystatechange = 开发者_如何学运维
function() {
if(this.readyState == 4) {
  //etc.. 
alert(req.responseText);
}
}; 
req.open('POST','/upload',true);
req.send(file);

Uploading files in XMLHttpRequest object is not supported for security reasons

EDIT: This is, however, possible with XMLHttpRequest 2

function upload(blobOrFile) {
  var xhr = new XMLHttpRequest();
  xhr.open('POST', '/server', true);
  xhr.onload = function(e) { ... };

  // Listen to the upload progress.
  var progressBar = document.querySelector('progress');
  xhr.upload.onprogress = function(e) {
    if (e.lengthComputable) {
      progressBar.value = (e.loaded / e.total) * 100;
      progressBar.textContent = progressBar.value; // Fallback for unsupported browsers.
    }
  };

  xhr.send(blobOrFile);
}

upload(new Blob(['hello world'], {type: 'text/plain'}));