- You can't delete the second post yourself. You can delete its content, but to delete the whole post you have to ask a moderator - like you've done.
- You can delete the attachment yourself. The way is more or less the same like you've attached it: Edit -> Go advanced -> Manage attachments -> Scroll down a little bit to: remove
- In your case I would let "live" the second post as it contains additional information about the changes of version 1.1 but delete the second attachment but it's of course your business what you're doing with your posts.