Add Filter to Media with a Taxonomy

gistfile1.txt

// register new taxonomy which applies to attachments
function reach_add_media_taxonomy() {
    $labels = array(
        'name'              => 'Media Type',
        'singular_name'     => 'Media Type',
        'search_items'      => 'Search Media Types',
        'all_items'         => 'All Media Types',
        'parent_item'       => 'Parent Media Type',
        'parent_item_colon' => 'Parent Media Type:',
        'edit_item'         => 'Edit Media Type',
        'update_item'       => 'Update Media Type',
        'add_new_item'      => 'Add New Media Type',
        'new_item_name'     => 'New Media Type Name',
        'menu_name'         => 'Media Type',
    );
 
    $args = array(
        'labels'                     => $labels,
		'hierarchical'               => true,
		'public'                     => true,
		'show_ui'                    => true,
		'show_admin_column'          => true,
		'show_in_nav_menus'          => true,
		'show_tagcloud'              => false,
		'show_in_rest'               => true,
    );
 
    register_taxonomy( 'media_type', 'attachment', $args );
}
add_action( 'init', 'reach_add_media_taxonomy' );

// Display a custom taxonomy dropdown in admin:
if ( ! function_exists('andrews_filter_post_type_by_taxonomy') ) {
function andrews_filter_post_type_by_taxonomy() {
global $typenow;
$post_type = '{attachment}';
$taxonomy = '{media_type}';
if ($typenow == $post_type) {
$selected = isset($_GET[$taxonomy]) ? $_GET[$taxonomy] : '';
$info_taxonomy = get_taxonomy($taxonomy);
wp_dropdown_categories(array(
'show_option_all' => __("Show All {$info_taxonomy->label}"),
'taxonomy' => $taxonomy,
'name' => $taxonomy,
'orderby' => 'name',
'selected' => $selected,
'show_count' => true,
'hide_empty' => false,
// 'hierarchical' => 1,
'value_field' => 'slug',
));
};
}
add_action('restrict_manage_posts', 'andrews_filter_post_type_by_taxonomy');
}

Here is a slightly different method, possibly more efficient as it doesn't use the global variable

// Display a custom Types dropdown in the media library
if ( ! function_exists('filter_media_library_cats') ) {
    function filter_media_library_cats( $post_type, $which ) {

    //Apply this only on a specific post type
    if ( 'attachment' !== $post_type )
        return;

    //A list of taxonomy slugs to filter by
    $taxonomies = array( 'media-category' );

    foreach ( $taxonomies as $taxonomy_slug ) {

        //Retrieve taxonomy data
        $taxonomy_obj = get_taxonomy( $taxonomy_slug );
        $taxonomy_name = $taxonomy_obj->labels->name;

        //Retrieve taxonomy terms
        $terms = get_terms( $taxonomy_slug );

        //Display filter HTML
        echo "<select name='{$taxonomy_slug}' id='{$taxonomy_slug}' class='postform'>";
        echo '<option value="">' . sprintf( esc_html__( 'Show All %s', 'text_domain' ), $taxonomy_name ) . '</option>';
            foreach ( $terms as $term ) {
                printf(
                    '<option value="%1$s" %2$s>%3$s</option>',
                    $term->slug,
                    ( ( isset( $_GET[$taxonomy_slug] ) && ( $_GET[$taxonomy_slug] == $term->slug ) ) ? ' selected="selected"' : '' ),
                    $term->name,
                    $term->count
                    );
                }
        echo '</select>';
        }
    }
    add_action( 'restrict_manage_posts', 'filter_media_library_cats' , 10, 3);
}