A schematic file is used to save files without taking much space at your PC. A Schematic file cant be loaded by Minecraft itself, you also cant save it as schematic files in minecraft

MCEdit uses .Schematic extension, you must use MCEdit to put the schematics in your world. Your world will be laggy first but after a long time it will become normal

-Rickydaan (MinecraftForums, &Ingame Name)