There is only one file type that uses the .UPX extension:
Ultimate Packer for eXecutables File
File created by Ultimate Packer for eXecutables (UPX), a program used to compress executable files; contains a packed executable file, such as an .EXE file, that has compressed at a high ratio; used to reduce the file size of executables for transport via email or external media.
Ultimate Packer for eXecutables automatically unpacks UPX files when they are executed. On some operating systems, it stores the unpacked file in memory using the in-place technique. On other operating systems, it creates a temporary file using the extraction technique.
NOTE: UPX files rarely use a ".upx" file extension. Rather, they most often use their original file extension after being compressed.
Updated: October 13, 2010