Drupal Multiupload Filefield Widget conflict with mutiselect ajax dropdown - php

Question

I have created a multiupload field in my node using Multiupload Filefield Widget. I am using the code below to alter the default node form.
There is an ajax based dependent dropdown field in the form i.e city dependent on country.When I select a single country the form work fine. But when I select multiple values form the country list I get the follwing error:
An illegal choice has been detected. Please contact the site administrator.
I get this particular error when there is the multiupload form element present. When I remove the element from $form its works fine with multiple selected values.
Please help me out to solve this problem.Thanks in advance!
function mymodule_form_alter(&$form, &$form_state, $form_id) {
switch ($form_id) {
case 'mynode_node_form':
$country_list = load_countries();
$selected_country = (isset($form_state['values']['country'])) ? $form_state['values']['country'] : key($country_list);
$form['countries'] = array(
'#type' => 'select',
'#title' => t('Select Your Country'),
'#options' => $country_list,
'#default_value' => $selected_country ,
'#ajax' => array(
'callback' => 'city_dropdown_callback',
'wrapper' => 'city_wrapper',
),
'#multiple' => TRUE,
'#required' => TRUE,
);
$cities_list = load_cities($selected_country);
$selected_cities = (isset($form_state['values']['cities'])) ? $form_state['values']['cities'] : key($cities_list);
$form['cities'] = array(
'#type' => 'select',
'#title' => t('Select Your City'),
'#prefix' => '<div id="city_wrapper">',
'#suffix' => '</div>',
'#options' => $cities_list,
'#default_value' => $selected_cities,
'#multiple' => TRUE,
'#required' => TRUE,
);
}
}
function city_dropdown_callback($form,$form_state){
return $form['cities'];
}