The following patch allow to add an attachment on a mailing.
The code might contain not that useful stuff, but the wizard is a bit too voodoo for me, I might have left things out of misunderstanding....
1) Once added, no way of deleting the attachement, beside copying the mailing.
2) When copying an event, doesn't copy the attachment.
Useful for my own usage IMMV
$svn diff CRM/Mailing > /tmp/php.diff
$svn diff templates/CRM/Mailing > /tmp/template.diff