The Free On-line Dictionary of Computing (30 December 2018):

file type

    The kind of data stored in a file.  Most modern
   operating systems use the filename extension to determine
   the file type though others store this information elsewhere
   in the file system.  The file type is used to choose an
   appropriate icon to represent the file in a GUI and the
   correct application with which to view, edit, run, or print
   the file.

   Different operating systems support different sets of file
   types though most agree on a large common set and allow
   arbitrary new types to be defined.

   See also MIME.

   FileInfo.com - The File Extensions Resource
   (http://fileinfo.com/)

   (2009-03-27)