The MS-DOS compression format uses the LZSS algorithm to compress a single file into another single file,
replacing the last character in the filename with an underscore. It is implemented via the MS-DOS commands COMPRESS.EXE and EXPAND.EXE.
A DOS compressed file will be named file.**_, so a .exe will look like file.ex_, a DOS compressed .jpg will look like file.jp_ etc.
https://www.cabextract.org.uk/libmspack/doc/szdd_kwaj_format.html