If McCain told the FBI that he couldn’t confirm the validity when he gave it to the FBI, then I don’t think McCain did wrong.
If we really had a compromised President of either party and a senator became aware of information, we would want them to send it to the FBI wouldn’t we?
But you have to question both the judgement of McCain and the FBI to think that this is blackmailable material.
While I have little use for that idiot McCain, he probably did the correct thing here. What else should he have done?
Now provided he was not the leak source to the media, than I cannot fault McCain for his actions here.