title: show alert box when user select non accepted file in danialfarid/ng-file-upload plugin
body: <ul>
  <li><p>I am using <a href="https://github.com/danialfarid/ng-file-upload" rel="nofollow">danialfarid/ng-file-upload</a> plugin ,I set the ngf-select
  directive to .doc,.pdf extension. Its working fine.</p></li>
  <li><p>Problem is if a user select .png or jpg file, I want to show an alert
  box to user that this file type are not accepted. Thanks for in
  advance.</p></li>
  </ul>