Hi Pancho Perez
I know that the topic is for 3 years ago and I don't know whether you find a solution to this problem or not.
But as I was searching for solution and I could not find anything on the web, I solved it on my own and I want to share it here.
The solution is very simple. WordPress registers the default post types (post,page,attachment and etc) with arguments that don't allow us to have archive pages for these post types.
you should add archive-{post_type}.php template files to your template folder (archive-post.php or archive-attachment.php) and then add this code to your functions.php file:
function dt_change_default_post_type_args($args, $post_type){
if($post_type == 'attachment'){
$args['has_archive'] = true;
$args['rewrite'] = array('slug'=>'media');
}
if($post_type == 'post'){
$args['has_archive'] = true;
$args['rewrite'] = array('slug'=>'blog');
}
return $args;
}
add_filter( 'register_post_type_args', 'dt_change_default_post_type_args', 10, 2 );
you can change rewrite argument to have your own slug or just set it boolean true to use post type default slug.
at last remember to save your permalink settings again after saving the functions.php file.