file_picker.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>TEST</title>
  <style>
    body {
      font-family: Arial, sans-serif;
      margin: 20px;
    }
    .info {
      margin-bottom: 20px;
    }
  </style>
</head>
  <form>
    <input type='file' id='picker'>
</form>
<body>
  <div class="info" id="info"></div>
  <script>
  
  document.getElementById('picker').addEventListener('change',
    function(e) {
        const reader = new FileReader();
        reader.onload = function(e) {
            alert(e.target.result);
        };
        reader.readAsText(e.target.files[0]);
    });

</script>

</body>
</html>