Domian Mapping without a Plug in

I’d like to resurrect this topic http://wordpress.org/support/topic/simple-domain-mapping-without-any-plugins-wordpress-30?replies=14

Because,
I’ve followed the advice and gotten the subdomain mapped correctly but I’m finding that my media library isn’t working. When I upload a file it appears to work then immediately I get the red x and the file isn’t there. What I’m looking for is where the subdomain things the media library is and how do it set it?

I created a domain as this http://bridge2thecloud.b2bmsp.com and once it was installed I changed it in WP to http://bridge2thecloud.com and made the setting changes to the wp-config file recommended above all works except media uploads. (I chose subdomain not subdirectory)

Thanks!