title: Vaadin : Upload button listener
body: <p>How can I get the upload button listener of component upload? This is because I want to validate other fields upon clicking the upload button. I've tried adding...</p>
  
  <pre><code>upload.addListener(new Upload.StartedListener() {});
  </code></pre>
  
  <p>It only works if there is already a chosen file but if the file is blank, it didn't. </p>