Open the editor and place a unit that has a weapon with attachements and a backpack, eg. a Blufor engineer (B_engineer_F).
Preview the mission, hit escape and type in the debug console: onEachFrame { hint str loadBackpack player; }; and execute locally.
Open your inventory, empty your backpack, now drag and drop your primary weapon in your backpack and note the load displayed in the hint.
Now take the weapon back in your primary slot, drag and drop each attachement (laser, opitc and magazine) inside your backpack and finally add the weapon to the backpack.
Note the difference with the load displayed before even though there are exactly the same items inside your backpack.