The [attach] bbcode is not on the bbcodes help page. Seems like it should be there for completeness, even though it is generally inserted by using the button when uploading images.
Yay! I'm hoping that eventually the page will be generated automatically and that we won't have to manually edit it each time we add a bbcode. You could add a bbcode option that hides it on the help page for bbcodes that are too complex to explain there or "secret" bbcodes.