UMAP

UMAP is an exclusive format to Unreal Engine 4 games. It contains map data and some game system information that makes the game functional. They cannot be previewed on UE Viewer nor loading straight into Unreal Engine 4 because they are "cooked". In some cases, these games tend to use "Level Streaming" which allows you to replace those .umap with your custom ones. Unlike .uasset, these files can be renamed and replace the other umap files without any issues.