This handy little editor extension allows you to use an image file of any type recognized by Unity as a height map and create new Terrain Data with it. (.png, .jpeg, etc.)
It is recommended, but not required, to use grayscale in the image to use. White pixels and brighter areas in the image signify higher elevations in the terrain, and black and darker areas signify the lower points in the terrain.
You can now apply smoothing when creating a terrain from a height map!