StuffIt X
From Just Solve the File Format Problem
StuffIt X is a newer format released by the makers of StuffIt. While the earlier StuffIt format and program were Mac-specific, StuffIt X has cross-platform features (for instance, storing file attributes from Windows and Unix directories) and is supported in software released for both Mac and Windows platforms. Like its predecessor, the StuffIt X format is proprietary.
Identification
Known files begin with "StuffIt!". According to The Unarchiver source code, they can also begin with "StuffIt?".
Compare to StuffIt#Identification.
