如何在php中获取数据blob？

I use recorder.js and I can't upload to server audio blob. My javascript function is

console.log(blob);
var fd = new FormData();
fd.append('fname', 'test.wav');
fd.append('data', blob);
$.ajax({
  type: 'POST',
  url: 'upload.php',
  data: fd,
  processData: false,
  contentType: false
  }).done(function(data) {
     console.log(data);
});

My upload.php is

<?php
$size = (int) $_SERVER['CONTENT_LENGTH'];
echo $size;
if (isset($_POST["data"])) echo " YES :D";
else echo " NO :((";

My console is: 475457 NO :(( Why $_POST["data"] is empty? Help me. Thank you.

You can try to use : "$_FILES" retrieve