Modal form file upload is not working
Make sure you're implementing the right Drupal API syntax which requires $form_state to be the reference to the original array, so it can be properly modified through certain hooks.

For example if you're using form_load_include() in hook_form_alter(), make sure you're following the right API syntax which is:

hook_form_alter(&amp;$form, &amp;$form_state, $form_id)


(note the &amp;) otherwise form_load_include() can't perform alterations of your current state of the form ($form_state).