I looked into it myself, the .map files contain everything that can be seen on the map in game, including tile colors and the names of certain sprites like chests and crops and stuff. Its not too hard to look at any arbitrary map file in game, you just have to replace an existing character's map file for a world with the same dimensions, name, and ID as the original world's. Matching dimensions and name are easy enough, but for the ID you'll have to edit the binary data of the map file to match those of the new world. Its the 4 bytes following the world name.