The kind of data stored in a file. Most modern operating systems use the filename extension to determine the file type though some 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.