global $post;
add_action( 'template_redirect', 'attachment_redirect' );
function attachment_redirect(){
 global $post;
 if( is_category(18) ) { // 1 yerine kategorinizin ID'sini yazın
 if ( is_attachment() && isset($post->post_parent) && is_numeric($post->post_parent) && ($post->post_parent != 0) ) :
    wp_redirect( get_permalink( $post->post_parent ), 301 );
    exit();
    endif;
}  
}