The diagonals bisect one another perpendicularly forming four right triangles. So the legs of these triangles have lengths 16/2=8cm and 12/2=6cm.
The hypotenuses are the sides of the rhombus, so the length of each side is √(8²+6²)=√(64+36)=√100=10cm, making the perimeter 4×10=40cm.