change_upload_dir

add_filter( 'upload_dir', 'change_upload_dir' );
function change_upload_dir( $args ) {
    $id = $_REQUEST['post_id'];
    if( get_post_type( $id ) == 'downloads' ) {
      $args['basedir'] = wp_normalize_path( WP_CONTENT_DIR . 'downloads' );

      $args['baseurl'] = content_url() . '/downloads';

      $args['path']    = $args['basedir'];

      $args['url']     = $args['baseurl'];

      $args['subdir']  = '';
      return $args;
    }
}