After few hours of gaming today I finished going through the main story in story mode. Game completion was around 57%. After spending my hard earned studs on some un-lockable characters I saw that game completion raised to 60,2% and at the same time the attachment popped. :)
Now "all" I need to do in this game is go through the levels in Free-play mode and find all the mini-kits and power bricks and work my way closer to the 100% completion. And go through the levels without dying... that's the hard part. Wish me luck!