title: How to check if file field is empty in codeigniter?
body: <p>I have a form with a file field called <code>image</code>, but this field is not required.<br>
  When user don't choose any file in form, the <code>do_upload()</code> always return a error.<br>
  How can I check if user chosen a file before perform the upload action in my controller?</p>