This is a program that examines a file and attempts to determine what it is. It is similar to the posix file(1) command but gives more detail and recognizes a wider variety of file types. (There are occasions when whatis recognizes that file could give more information, and calls the latter program.)
Note: This program has an unfortunate name collision with the posix program, whatis(1). You can easily rename the program when you download it.
Linux version (Version 2.130, October 16, 2021)
Example output:
$ whatis index.html WHATIS - Version 2.58 Copyright 1989-2011, Glenn Story File: /home/glenn/websites/personal/tools/index.html File-system analysis indicates EXT2 or EXT3 file system File-mode analysis indicates regular file File extension (.html) indicates Hyper-Text Markup Language file File analysis indicates DOS/Windows ASCII Text file Text analysis indicates HyperText Markup Language (HTML) file