title: allow click in a button if image is selected
body: <p>I have a form to upload image.</p>
  
  <p>I want to disable this button and enable it only when user selected some file. Any ideas how can I make it work?</p>
  
  <pre><code>&lt;form action="/upload.php" id="upload" method="POST"&gt;
      &lt;input type="file" name="photo" id="photo" class="file_input"&gt;
      &lt;input type="submit" value="Upload" id="upload_btnb" disabled&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>how to enable button when user select a file?</p>