I don't think this should be a Constance setting. Changing the attachment host seems like a major enough thing to require a code push to support.

Also, since it's a Constance setting, need to add a note to vagrant installation docs to change it to point to `mdn-local`. But, that's another thing that could be obviated by moving it out of Constance and into plain old settings: It could just be set in `settings_local` and managed in Puppet.