chromium/chrome/test/data/web_apps/file_handler_action.html

<!DOCTYPE html>
<html>
<head>
  <link rel="icon" href="basic-48.png">
</head>
<body>
  <h1>Web app with file handlers - Handle launch</h1>
  <script>
    if ("launchQueue" in window) {
      window.launchQueue.setConsumer(launchParams => {
        console.log("Launched with: ", launchParams);
      });
    }
  </script>
</body>
</html>