There is only one file type that uses the .UPK extension:
Unreal Engine 3 Package
File used by Unreal Engine 3, a free gaming engine used to develop popular games such as Unreal Tournament 3, Mass Effect, Mirror's Edge, and BioShock; serves as a file container which includes various types of game files; may contain 3D models, textures, maps, music files, game settings, and other files.
UPK files are used to load on-demand content while playing a game. They can be created and edited using the Unreal Development Kit (UDK), which is the free edition of Unreal Engine 3. However, UDK is no longer supported by Epic Games.
Updated: February 26, 2018