If I play the Miner of the Iron Hills, it say " After Miner of the Iron Hills enters play, CHOOSE and discard 1 Condition attachment from play.".

But what happens if I have Unexpected Courage or other "good" Condition attachments in play?

If someone says to me: "choose and discard a card" I think I have to do it.

Maybe they didn't want to get such a bad effect on my own attachments, but the wording could be better, as " you MAY choose and discard 1 Condition attachment from play".

Miner's ability is a Response, which means it is your choice whether or not you use the ability. Responses are not mandatory.

thank you! I missed it.

If you had to choose something it would be listed as Forced...