I had nothing to do with it, I swear. It seems ActiveBoard really likes sneaking new features into forums hosted on their servers without notifying the administrators.
However, please don't feel intimidated by it. I just tried it out and it seems simple enough, and is actually quite an improvement. Now we can select multiple files at once, instead of one-at-a-time. I always wanted this!
Have a look:
And one more thing...
The attachment thumbnails in posts are now smaller. I guess it's to keep from board clutter, but I personally liked it the way it was.