function atelier_add_iCat_taxonomy() {
$labels = array(
    'name'              => 'iCats',
    'singular_name'     => 'iCat',
    'search_items'      => 'Search iCats',
    'all_items'         => 'All iCats',
    'parent_item'       => 'Parent iCat',
    'parent_item_colon' => 'Parent iCat:',
    'edit_item'         => 'Edit iCat',
    'update_item'       => 'Update iCat',
    'add_new_item'      => 'Add New iCat',
    'new_item_name'     => 'New iCat Name',
    'menu_name'         => 'iCat',
);

$args = array(
    'labels' => $labels,
    'hierarchical' => true,
    'query_var' => 'true',
    'rewrite' => 'true',
    'show_admin_column' => 'true',
);

register_taxonomy( 'iCat', 'attachment', $args );

function atelier_add_image_category_filter() {
$screen = get_current_screen();
if ( 'upload' == $screen->id ) {
    $dropdown_options = array( 'show_option_all' => __( 'View all categories', 'iCats' ), 'hide_empty' => false, 'hierarchical' => true, 'orderby' => 'name', );
    wp_dropdown_categories( $dropdown_options );
}