The "Forest spider" card (core 96), in the shadow effet area, says: " The defending player must choose and discard an attachment card which controls ". It means also for an attachment card that is in the hand of the player or it is valid only for an attchment card that is on the play field? I ask this because the page nr. 25 of the english rules book says: " A player controls all cards that he owns ". Please, can someone help me? Thanks in advance.

No, effects don't interact with cards out of play (deck, discard or hand) unless they specifically state so (Fortune or Fate), so the attachement has to be in play.

Grazie! Thank you very much!