content.html

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>File Manager</title>
</head>
<body>
<button onclick="openFileManager(callback)">Open File Manager</button>
<div class="app">
    <p>Hello</p>
</div>
<p>Hello</p>
<script src="dist/build.js"></script>
<script>
  function callback(list) {
    if(list.length !== 1) {
      console.log("Select one file");
      return false;
    }
    return true;
  }
</script>
</body>
</html>