I would guess when they ask to see it, they want to assess how serious it is. The comment about doing it for attention is really invalidating though. Therapists are becoming more educated about self-injury now, but unfortunately they don't all know how to work with it effectively.

Maybe you could ask to talk about all of the possible reasons why you SI? Usually there are several. You could go in with a list of reasons why people do it, and mark the ones that you feel apply to you.