[BUG] Asset Manager sends empty request when uploading image

Description: GrapesJS editor is initialized in an editor.html file as follows:

  var editor = grapesjs.init({
    container: '#gjs',
    height: '100%',
    fromElement: true,
    plugins: ['gjs-preset-newsletter'],
    assetManager: {
      autoAdd: 1,
      upload: 'http://localhost:9001/uploadimage.php',
      uploadName: 'upload',
    },
  });

localhost:9001 is a local IIS server running PHP5. The content of uploadimage.php is as follows:

<?php
$logfile = fopen("testlog.txt", "a+");
$req_dump = print_r($_FILES, TRUE);
fwrite($logfile, $req_dump . PHP_EOL);
fclose($logfile);

Since images uploaded from GrapesJS is expected to be in the $_FILES array, I dump all of its content to a log file everytime a request reaches this page. The testlog.txt file always gets

Array
(
)

each time I upload an image from the Image element found in gjs-preset-newsletter plugin, indicating that Asset Manager is sending empty requests to the server. File upload is enabled in my php.ini:

file_uploads = On
upload_max_filesize = 2M
max_file_uploads = 20

And the test image file is way below PHP's limit. To be exact, I used GitHub_Logo.png found in a package here for testing. Am I doing something wrong?

OS: Windows 10 GrapesJS version: v0.14.6 Browser: Chrome 65