instance_eval was the wrong approch for this.  It was evaluating the pub...

instance_eval was the wrong approch for this.  It was evaluating the public_url on one and only one instance ever... the instnace that was the base on first go around.  This resulted in badness in the form of re-using the same base for all calls to get me my public url.  commited a better approach to call the method directly on the provided attachment