i seem to remember i asked this question and the answer is no- you never can discard attathents from your hand. i will try and find it to see if i remember correctly.

taking your quites CJMatos, then there seems to be a bit of a contradiction in the rules....

Maybe only contradiction in the words, but not in the meaning.

You can see that the end of paragraph it was speaking about the "cards in game": ..."when a card leaves play"

So when a shadow effect says "discard an attachment you control," are you discarding cards from your hand?

Good question....

I've only quoted the rules, but i stand corrected with the rulling from Caleb.

I think that the "a player control all cards that he owns" is implicitly limited to cards that are in play.