Drupal Multiupload Filefield Widget与mutiselect ajax下拉列表冲突

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'];
}