Hi

The following shadow effect says:

Shadow: Choose and discard 1 attachment you control. Discarded objective cards are returned to the staging area. (If this attack is undefended, discard all attachments you control.)

Is it correct to discard an attachment in my hand? , I don't understand if the effect refers exclusively to the attachments that have been already played.

Regards,

You have to discard one that is in play. You only "control" a card if it's in play. The core set manual is very sloppy about explaining this, but that's how it is -- you "own" cards in your hand, but you don't "control" them.