app.py

# ... snip

def generate_upload_signed_url_v4(bucket_name, blob_name, *, origin=None):
    storage_client = storage.Client()
    bucket = storage_client.bucket(bucket_name)
    blob = bucket.blob(blob_name)
    headers = {}
    if origin:
        headers['origin'] = origin
    return blob.generate_signed_url(
        version="v4",
        expiration=datetime.timedelta(minutes=60),
        method="POST",
        headers=headers
    )

@app.route('/uploads/<string:upload_id>', methods=['GET'])
def upload(upload_id):
    bucket_name = os.environ['GOOGLE_STORAGE_BUCKET_NAME']
    file_name = f'uploads/{request.environ["iap_user"]["username"]}/{upload_id}'
    origin = cors_origin(request)
    signed_upload_url = generate_upload_signed_url_v4(bucket_name, file_name, origin=origin)
    return render_template('upload.html', upload_id=upload_id, signed_upload_url=signed_upload_url, current_user=request.environ['iap_user'])

# ... snip

upload.html.jinja2

<script src="https://releases.transloadit.com/uppy/v2.1.0/uppy.min.js"></script>
<script type="text/javascript">
window.addEventListener('DOMContentLoaded', (e) => {
  const progressBar = document.getElementById('progressbar');
  const dropzone = document.getElementById('dropzone');
  const decryptUrl = '{{ url_for("decrypt_upload", upload_id=upload_id) | safe }}';
  var uppy = new Uppy.Core({
    autoProceed: true,
    allowMultipleUploadBatches: false,
    debug: false,
    restrictions: {
      maxNumberOfFiles: 1,
      allowedFileTypes: ['text/csv', '.csv']
    },
  });
  uppy.use(Uppy.XHRUpload, {
    method: 'POST',
    endpoint: '{{ signed_upload_url | safe }}',
    getResponseData: () => { return {url: decryptUrl} }
  });
  uppy.use(Uppy.DragDrop, { target: '#dropzone' });
  uppy.on('upload', () => {
    dropzone.style.display = 'none';
  });
  uppy.on('complete', () => {
    window.location.href = decryptUrl;
  });
  uppy.on('progress', (progress) => {
    progressBar.style.width = `${progress}%`;
  });
});
</script>