XAR (Executable Archive)
From Just Solve the File Format Problem
XAR (eXecutable ARchive), or XARs, is a self-contained executable/archive file format developed by Facebook.
A XAR file can contain a program (e.g. a Python script), along with that program's dependencies, and any other required data. When executed, it mounts a SquashFS filesystem.
Identification
A XAR file apparently begins with ASCII "#!/usr/bin/env xarexec_fuse
" (see shebang), or something similar.