This is the specifications for the archive file which is handled by the Game Extractor plugin Archive_UE3_805.

Endian Order:	Little Endian
Compression:	ZLib, LZO, LZX
Encryption:	None

This format is used by the following Games:

Batman: Arkham City

*.upk

These are the accompanying notes and further information:

Unreal Engine 3 (Version 805)

Game Extractor is able to read files of this type, as well as files from thousands of other games. To give it a try, download Game Extractor (Basic Version) for free, and see what it can do.

For further information on this format, refer to the source code on our GitHub repository.