an easy way to use attachment. no trigger needed. use the ability sphere of the bloodmage, edit it like you want and give the ability to the unit of your choice, if you want exemple, if the unit pick a item sword exemple, you want see the weapon on his hand, give the ability to the item and when the hero will pick the item the sword will apear in his hand.
this is how you have to set the ability sphere, now it set whit 5 attachment, but if you want use it only to add a weapon, set it whit only 1 on the hand of your choice.