treeprop.png is used for the currently quite broken "farmesh" feature. To enable it, put this in minetest.conf:
Your phone or window isn't wide enough to display the code box. If it's a phone, try rotating it to landscape mode.
Note that farmesh_trees and farmesh_distance options have been removed since a while ago -- farmesh trees are always drawn and distance is 512 or so. Unfortunately, the farmesh tends to not be representative of the world like it should be, does not account for buildings or stuff generated in Lua, and is also very slow (less than 20 FPS here!).