JSON_encode在文件编码上返回null

I'm trying to manipulate a uploaded song using PHP and JavaScript. To connect the two, I'm using AJAX.

HTML

<form enctype="multipart/form-data" method="POST" action="fileUpload.php" id="uploadForm">
  <input type="hidden" name="Max_Size" value="2048">
  <input type="file" name="uploadedSong" accept=".mp3"><br>
  <button type="submit">Submit</button>
</form>

JS:

formSongUpload.addEventListener('submit', function(){
  let requisition = new XMLHttpRequest();
  requisition.onreadystatechange = manipulateThisSong(//Uploaded Song Goes Here);
  requisition.responseType = 'json';
  requisition.open('POST', 'fileUpload.php');
  requisition.send();
  //Temporary alert
  alert(requisition.response);
})

PHP:

<?php
  $file = $_FILES['uploadedSong']['name'];
  //Not verifying the file size here, just trying to send the upload
  echo(json_encode($file));
?>

However, the response is recieving null. Why does this happen and how can I resolve it?