Is it safe to warm bread in the oven with plastic wrap?
Unless the plastic specifically states that it's ovenproof, do not heat it in the oven.

You can just heat the unwrapped loaf in the oven (I usually do), but it will make the crust crustier.  Placing the loaf in a paper bag will reduce this (crusting) effect.  The paper bag can be dampened to reduce the effect even further.