wordpress: Add custom field to gallery popup Wordpress

gistfile1.php

<?php

/* For adding custom field to gallery popup */
function ime_autora_field($form_fields, $post) {

    $form_fields["ime-autora"] = array(
        "label" => __("Autor"),
        "input" => "text",
        "value" => get_post_meta($post->ID, "_ime-autora", true),
                "helps" => __("Ime autora slike"),
    );
   return $form_fields;
}
// now attach our function to the hook
add_filter("attachment_fields_to_edit", "ime_autora_field", null, 2);

function ime_autora_field_save($post, $attachment) {
    if( isset($attachment['ime-autora']) ){
        update_post_meta($post['ID'], '_ime-autora', $attachment['ime-autora']);
    }
    return $post;
}

add_filter("attachment_fields_to_save", "ime_autora_field_save", null , 2);