A WordPress function to fix pagination when combining custom post types in a single loop.

fix_allcustomposts_pagination.php

<?php
/**
 * WordPress function to fix pagination when combining custom post types in a single loop.
 */
function fix_allcustomposts_pagination($qs)
{
    if( !isset( $qs['post_type'] ) && isset($qs['paged']) )
    {
        $qs['post_type'] = get_post_types($args = array(
            'public'   => true,
            '_builtin' => false
        ));
        array_push($qs['post_type'],'post');
    }
    return $qs;
}
add_filter('request', 'fix_allcustomposts_pagination');