index_js.html

<script>
document.getElementById('submit').addEventListener('click', _ => {
  const name = document.getElementById('name').value;
  const school = document.getElementById('school').value;
  const quote = document.getElementById('quote').value;
  const photo = document.querySelector('input[type=file]').files[0];
  const reader = new FileReader();
  reader.addEventListener('load', e => {
    const photoData = {
      filename: photo.name,
      bytes: [...new Uint8Array(e.target.result)]
    };
    document.getElementById('result').innerText = 'Loading...';
    google.script.run.withSuccessHandler(onSuccess).withFailureHandler(onFailure).createNewSlide(name, school, quote, photoData);
  });
  reader.readAsArrayBuffer(photo);
});

function onSuccess() {
  document.getElementById('result').innerText = 'Congrats, you are now Registered!';
}

function onFailure() {
  document.getElementById('result').innerText = 'An Error Occured. Please try again';
}
</script>