title: Check if specific input file is empty
body: <p>In my form I have 3 input fields for file upload:</p>
  
  <pre><code>&lt;input type=file name="cover_image"&gt;
  &lt;input type=file name="image1"&gt;
  &lt;input type=file name="image2"&gt;
  </code></pre>
  
  <p>How can I check if <code>cover_image</code> is empty - no file is put for upload?</p>