Zipx
From Just Solve the File Format Problem
Zipx refers to a ZIP file that uses advanced compression algorithms that are not universally supported. Among the possible algorithms are bzip2, LZMA, WavPack, and PPMd. The Zipx "format" originated with the WinZip software.
Disambiguation
This article is not about the ZipX compression software, although apparently ZipX does support Zipx files.