Wordpressのメディアにカテゴリーとタグを付ける

function.phpに以下を追加することで、メディア（attachment)にカテゴリーとタグを付けられます。

function yk_register_taxonomy() {
    // add category                                                                                                
    $labels = array(
                    'name' => _x( 'Attachment Categories', 'taxonomy general name'),
                    'singular_name'     => _x( 'Attachment Category', 'taxonomy singular name' ),
                    'search_items'      =>  __( 'Search Attachment Categories' ),
                    'all_items'         => __( 'All Attachment Categories' ),
                    'parent_item'       => __( 'Parent Attachment Category' ),
                    'parent_item_colon' => __( 'Parent Attachment Category:' ),
                    'edit_item'         => __( 'Edit Attachment Category' ),
                    'update_item'       => __( 'Update Attachment Category' ),
                    'add_new_item'      => __( 'Add New Attachment Category' ),
                    'new_item_name'     => __( 'New Attachment Category Name' ),
                    'menu_name'         => __( 'Attachment Category' ),
                    );
    $args = array(
                  'hierarchical' => true,
                  'labels'       => $labels,
                  'show_ui'      => true,
                  'query_var'    => true,
                  'rewrite'      => true,
                  );
    register_taxonomy('attachment_category', 'attachment', $args);
    // add tag                                                                                                     
    $labels = array(
                    'name'              => _x( 'Attachment Tags', 'taxonomy general name' ),
                    'singular_name'     => _x( 'Attachment Tag', 'taxonomy singular name' ),
                    'search_items'      =>  __( 'Search Attachment Tags' ),
                    'all_items'         => __( 'All Attachment Tags' ),
                    'parent_item'       => __( 'Parent Attachment Tag' ),
                    'parent_item_colon' => __( 'Parent Attachment Tag:' ),
                    'edit_item'         => __( 'Edit Attachment Tag' ),
                    'update_item'       => __( 'Update Attachment Tag' ),
                    'add_new_item'      => __( 'Add New Attachment Tag' ),
                    'new_item_name'     => __( 'New Attachment Tag Name' ),
                    'menu_name'         => __( 'Attachment Tags' ),
                    );
    $args = array(
                  'hierarchical' => false,
                  'labels'       => $labels,
                  'show_ui'      => true,
                  'query_var'    => true,
                  'rewrite'      => true,
                  );
    register_taxonomy('attachment_tag', 'attachment', $args);
}

add_action('init', 'yk_register_taxonomy');

プラグインを使うなら、Attachment Taxonomy Supportがおすすめです。