There is only one file type that uses the .WUX extension:
Compressed Wii U Disk Image File
A WUX file is a Wii U Disk Image .WUD file compressed using the Wii U image compression tool. It contains all the information about a game in the WUD format but is compressed to save space. WUX files can be opened by Cemu to emulate Wii U games on a computer.
WUX files are the compressed version of WUD files, which are typically created using Homebrew for Wii U to store game information dumped from a Wii U game. Since the game information is large in size (over 20 GB), multiple WUD files are created to store segments (around 2 GB) of the game. In order to reduce the size of a WUD file, the Wii U image compression tool can be used to compress the file and reduce it to a much smaller size, sometimes from sizes around 20 GB to 2-3 GB. The compression of the WUD file results in the WUX file.
You can decompress WUX files with the Wii U image compression tool, which results in a WUD file. You can also simply open a WUX file in Cemu by selectingand choosing the WUX file you want to open.
Updated: December 28, 2017