So my question is simple: If i use Expert Treasure hunter and i say attachment, then discarded card is Thror´s Golden Cup, do i add it to my hand or do i leave it in the discard pile?

You'll have to leave it in the discard pile.

Once the card is attached to a character, it is considered an "attachment" for the purposes of cards that target attachments. The card type, however, is still Treasure.

Ok than i have been played right, thanks again GrandSpeelen

I just thought I would add to this thread with some The Road Darkens info. I noticed that the Boon cards in TDR are actually "attachment" card type, so unlike the Hobbit saga Treasures, these things are compatible with cards like tactics Bofur.