This appendix lists many of the most common filename extensions that you'll find on your system, that you might download, or that you have received over the Internet.
Extensions were universally used on DOS and Windows 3.1 files, but Microsoft has gone to some difficulty to hide them in Windows Vista. This is unfortunate, because they play a major role in the way Windows decides what application will be used to open a file, as well as which files will be visible when opening files in a given application. Although direct associations are made between some files without extensions and the applications needed to open them, in most cases the association is between an extension and a Registry setting that tells the system what application to use. To enable the display of filename extensions, open Windows Explorer, go to Control Panel [Appearance and Personalization] Folder Options View tab, and turn off the "Hide extensions for known file types" option.
If you double-click on an unknown file type, the Open With dialog box appears, allowing you to make a new association. To subsequently change an association once it has been made, right-click on the file and select Open With, or select Properties and then click Change.
Table D-1 list many system extensions, but third-party applications use literally thousands of file formats. And you might be thrown off by an improperly named file, or by an application using a standard filename extension for a nonstandard purpose. You can open many of these file types only if you have the appropriate application. If all else fails, you can open just about any file in Notepad; although nontext files will look mostly like gibberish, you may get clues in some of the excerpts of readable text you'll see.
If you still can't find a file's type from its extension, there are several more good sources:
Microsoft has put together a simple "extension finder" web site, intended for use with the "Use the Web service to find the correct program" feature that appears when you try to open a file with an extension that Windows doesn't recognize. The address for this site is:
where {your extension} is the filename extension (without the dot) that you want to learn more about. For example, to find out about the .sit extension, you'd go to:
Use an ordinary search engine such as Google (http://www.google.com) to search for references to the filename extension. For example, to find out about the .hqx extension, try searching for "hqx file" (with the quotes).
Table D-1. Common filename extensions
Extension
Description
.$$$
Temporary file
.1st
ASCII text file (e.g., READ.1st)
.3ds
3D Studio file
.3gp, .3gpp
Video clip created on a cell phone or other mobile device
.3gr
Windows Video Grabber datafile
.8m
PageMaker Printer font with Math 8 extended character set
.8u
PageMaker Printer font with Roman 8 extended character set
.abf
Adobe Binary Font
.abm
Photo album
.abs
MPEG audio sound file
.aca
Microsoft Agent Character file
.ace
Ace Archiver compressed file
.acf
Microsoft Agent Character file
.ad
AfterDark screensaver
.adi
AutoCAD graphics
.adx
Archetype designer document
.afi
Truevision bitmap graphics
.afm
ATM Type 1 font metric ASCII data for font installer
.ai
Adobe Illustrator vector graphics
.ans
ANSI graphics; character animation
.aps
Microsoft Visual C++ file
.arc
Compressed file archive
.arf
Automatic Response File
.arj
Compressed file archive created by ARJ
.asc
ASCII text file
.ascx
Microsoft ASP.NET user control file
.asd
Word for Windows Autosave file
.asm
Assembly source code file
.asmx
Microsoft .NET Web Service file
.asp
Microsoft Active Server Page
.aspx
Microsoft ASP.NET file
.asx
Microsoft Windows Media Active Stream Redirector file
.asx
Windows Media Streaming video shortcut
.atm
Adobe Type Manager data/information
.au
Audio file used on older web pages
.avi
Video for Windows Audio Video Interleaved movie clip
.awm
Animation Works movie
.b3d
3D Builder file
.bak
Backup file (generic)
.bas
Basic source code file
.bat
DOS batch file
.bco
Bitstream Outline font description
.bdr
Microsoft Publisher Border
.bez
Bitstream Outline font description
.bga
Bitmap graphics
.bib
Bibliography (ASCII)
.bin
Binary file
.bit
Bitmap X11
.bkf
Microsoft backup file
.bkw
FontEdit mirror image of font set
.bm
Bitmap graphics
.bmf
Corel image file
.bmp
Windows bitmap
.bsc
Microsoft Fortran Pwbrmake object file
.bup
Backup file (generic)
.c
C source code file
.c++
C++ source code file
.cab
Microsoft installation archive Cabinet file
.cas
Comma-delimited ASCII file
.cc
C++ source code file
.ccb
Visual Basic animated button configuration
.cel
Autodesk AnimatorLumena Graphics
.cfg
Configuration file
.cgi
Common Gateway Interface script
.cgm
Computer Graphics Metafile vector graphics
.chd
FontChameleon Font descriptor
.chk
ChkDsk recovered data
.cif
Caltech Intermediate Format graphics
.cif
Easy CD Creator CD image file
.class
Compiled Java class file
.cls
C++ class definition file
.cmf
Creative Music File FM-music file
.cnt
Helpfile contents
.col
Microsoft Multiplan spreadsheet
.contact
Windows Contacts contact entry
.cpi
Colorlab Processed Image bitmap graphics
.cpi
DOS Code Page Information file
.cpp
C++ source code file
.cps
Colored PostScript files
.crf
Microsoft MASMZortech C++ cross-reference
.cs
C# source code file
.csv
Comma Separated Value text file format (ASCII)
.ctx
Microsoft online guides Course TeXt file
.ctx
Pretty Good Privacy RSA System cipher text file
.cvp
WinFax cover page
.cvs
Canvas graphics
.dat
Datafile (generic)
.dbg
Microsoft C/C++ symbolic debugging information
.dcr
Shockwave file
.dcs
CYMK format bitmap graphics
.ddb
Bitmap graphics
.dev
Device driver (old)
.dib
Device-Independent Bitmap bitmap graphics
.dic
Lotus Notes/Domino dictionary file
.dip
Graphics
.diz
Description In Zip description file
.dlg
Microsoft Windows SDK dialog resource script file
.doc
Microsoft Word document
.dot
Word for Windows template
.drv
Device driver (old)
.dvr-ms
Windows Media Center recorded video file
.dxf
AutoCAD Drawing Interchange File format vector graphics