The Unposter is used to grow mushrooms.
You can deposit the same items as with the composter, except instead of making compost, when a mushroom is within its range, it will consume the items inside to multiply that mushroom in the area around it.
The Unposter searches for and duplicates mushrooms in a 11x11 square area centered on itself.
Note that mushrooms, once picked up, cannot be placed back down. You will need to find naturally spawned mushrooms for the unposter to work.