Javascript - Problema con upload file

Salve ragazzi come da titolo ho un problema con l'upload del file. Posto il codice:
Form

<form id="file-form" action="handle_file_upload.php" method="POST">
  <input type="file" id="file-select" name="photos[]" multiple/>
  <button type="submit" id="upload-button">Upload</button>
</form>

.js

var form = document.getElementById('file-form');
var fileSelect = document.getElementById('file-select');
var uploadButton = document.getElementById('upload-button');
form.onsubmit = function(event) {
  event.preventDefault();

  // Update button text.
  uploadButton.innerHTML = 'Uploading...';

  // The rest of the code will go here...
}

// Get the selected files from the input.
var files = fileSelect.files;

// Create a new FormData object.
var formData = new FormData();

// Loop through each of the selected files.
for (var i = 0; i < files.length; i++) {
  var file = files[i];

  // Check the file type.
  if (!file.type.match('image.*')) {
    continue;
  }

  // Add the file to the request.
  formData.append('photos[]', file, file.name);
}

// Files
formData.append(name, file, filename);

// Blobs
formData.append(name, blob, filename);

// Strings
formData.append(name, value); 

// Set up the request.
var xhr = new XMLHttpRequest();

// Open the connection.
xhr.open('POST', 'handle_file_upload.php', true);

// Set up a handler for when the request finishes.
xhr.onload = function () {
  if (xhr.status === 200) {
    // File(s) uploaded.
    uploadButton.innerHTML = 'Upload';
  } else {
    alert('An error occurred!');
  }
};

// Send the Data.
xhr.send(formData);

.php

<?php
if(move_uploaded_file($_FILES['photos']["tmp_name"], "upload/".$_FILES['photos']['name'])) {

	echo 'Ok';
} else {
	echo 'NONE';
}

?>

In pratica succede che: il button resta fisso su Uploading... e non carica il file nella cartella upload/.
Dove sbaglio?

Grazie

Aggiungi:

enctype="multipart/form-data"

Al tag form.