load_file.html

<div>
  <!-- Include the Google CSS package -->
  <link rel="stylesheet" href="https://ssl.gstatic.com/docs/script/css/add-ons.css">

  <!-- You can also include your own CSS styles -->
  <style>
   form { margin: 40px auto; }
   input { display:block; margin: 20px; }
  </style>

  <!-- The function will be called after the Google Script has executed -->
  <script>
    function fileUploaded(status) {
        document.getElementById('myForm').style.display = 'none';
        document.getElementById('output').innerHTML = status;
    }
  </script>

  <!-- This is the actual HTML form -->
  <form id="myForm">
      <!-- Text input field -->
      <input type="text" name="myName" placeholder="GDK">

      <!-- File input filed -->
      <input type="file" name="theFile">

      <!-- The submit button. It calls the server side function uploadfiles() on click -->
      <input type="button" value="UpLoad" id="button"
        onclick="google.script.run.withSuccessHandler(fileUploaded).uploadTheFile(this.parentNode)">
  </form>

  <form id="closeForm">
    <input type="button" value="Close" onclick="google.script.host.close()" />
  </form>

  <!-- Here the results of the form submission will be displayed -->
  <div id="output"></div>

</div>