title: How to use FileUpload widget in jupyter lab?
body: <p>I want to use the <code>FileUpload</code> widget in jupyter lab.
  I have the following lines of codes in my notebook cell :</p>
  <pre><code>uploader = widgets.FileUpload()
  uploader
  </code></pre>
  <p>In jupyter notebook, the output of the cell is a clickable button that I can use to upload a file. In jupyter lab, the output is the following :</p>
  <pre><code>FileUpload(value={}, description='Upload')
  </code></pre>
  <p>Here's the info on the <code>uploader</code> object :</p>
  <pre><code>Type:           FileUpload
  String form:    FileUpload(value={}, description='Upload')
  File:           ~/miniconda3/envs/fastai2/lib/python3.7/site-packages/ipywidgets/widgets/widget_upload.py
  </code></pre>
  <p>Is it possible to make this widget work on jupyter lab? And if so how should I proceed ?</p>