Needs to be a .ZIP File.
EDIT: If your on Windows XP this is how to make it into a .ZIP file. Right-click BL, click Properties click Find Target, click Add-Ons, look for in, right-click it, move your mouse thing to send to, finaly hit Send to (Compressed) ZIP File.
EDIT2: Can you add a "all of the above" vote to the poll?