hold the backpack in your hands, have an attachable item inside an inventory slot, try dragging the attachable item (radio, chemlight, etc) to one of the slots that it does belongs to

Example: backpack in hands, radio inside inventory, try dragging the radio into the chemlight ghost icon of the backpack, the radio will disappear

also

backpack equipped, can only attach item if the item is on character hands on ground, if its inside inventory, it wont attach by dragging it over ghost icons in general, only if you drag it to it's own ghost icon

Video illustrating and explaining the issue
https://www.youtube.com/watch?v=-vUrLBO6qbU&feature=youtu.be