Switch Upload Directory for a Custom Post Type

add_filter( 'upload_dir', 'wpfolks_portfolio_upload_directory' );

function wpfolks_portfolio_upload_directory( $dirs ) {
    if ( 'portfolio' !== get_post_type() ) {
        return $dirs;
    }

    $dirs['subdir'] = '/portfolio' . $dirs['subdir'];
    $dirs['path']   = $dirs['basedir'] . $dirs['subdir'];
    $dirs['url']    = $dirs['baseurl'] . $dirs['subdir'];

    return $dirs;
}