A .mesh file is an object file. Together with .material files, they make up the rocks and scenery in-game. x3 And, test map? o: If you downloaded it from the web (and it's in .fho, .fhm, .fhs format) you might want to put it in Exports. If it's a folder with a .map file, place it in Terrains.
These folders are in C:\FeralHeart\media.
And, don't try to open them -- the game reads them automatically. But, if you haven't deleted any, there's no need to worry about that. c: