File names have three parts, the name, extension, and directory name.
Extensions are 0 to 3 letters or digits.  The file extension is often used
to identify the different kinds of files.  Some common extensions and
their meanings are:

REL	relocatable binary file
DMP	core image file
F4	Fortran Source
SAI	Sail source
FAI	FAIL source
PSC	Pascal source
LSP	Lisp source
LST	listing output file
DOC	some documentation file
MSG	MAIL message file
DIS	distribution list for MAIL
PUB	PUB source
MSS	Scribe source
TEX	TeX source
MF	Metafont source
XGP	file prepared for Xerox Graphics Printer
DVI	device-independent document (TeX output)
PRE	Press file (for Dover)
IMP	imPress file (for Imagen)
PS	PostScript file (for Laserwriter)
FNT	XGP font file
TFM	TeX Font Metric file
TMP	temporary file for some program
RPG	information saved from RPG commands (COMPILE, etc.)
TXT	ASCII text file other than above

On the program documentation areas [S,DOC] and [UP,DOC], the file
extension is usually used to indicate the programmer name of the
author of the file.