jelonertz said:
Until we get an official answer I will keep on playing this card the way is only playable on your own cards, following the more restrictive rule of "the controller of the card chooses where (amongst his three zones) the card is placed".
Then you will be playing incorrectly. I have no doubt that if an attachment reads to play on a target unit, then you can play it on any target unit. If the card reads to play it on a unit you control, then you can only play it on a unit that you control. Feel free to wait 6 months for a FAQ to confirm this.