I played passage trough Mirkwood and shadow card was dealt to me what was forest spider: Defending player must choose and discard 1 attachment he controls.

So what if i dont defend against this attack?

I think you should just read it as "the player that Forest Spider is attacking"

If the card reads "defending player," the shadow effect needs to be resolved whether or not the attack is defended. If it reads "the defending character," it will only be resolved if the attack is defended. For undefended attacks, there is no defending character, but you are still the defending player.

Thanks