It might be more accurate to say that the 'attachment' variable isn't designed to be used in that way - it only accepts one attachment. One approach you might want to consider is to have a 'select' variable called 'number of attachments' that accepts a numerical value between zero and a maximum number, like 5. You then create 5 'attachment' variables on the item/record producer, and use 5 UI policies to show/hide each variable... so there's a UI policy that detects if the variable is one or greater, and shows attachment variable number 1. A second UI policy checks if the value is 2 or greater, and if so, displays the second attachment variable. Repeat up to the maximum.
Obviously, it may be possible to create a custom version of this widget that would allow any number of attachments, but the OOTB 'attachments' functionality on the form as a whole already does this, and these variables have a range of other, powerful benefits like the ability to validate file type that you don't get, so this is the approach I'd recommend.
