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:
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 |
.emf | Enhanced Metafile graphics |
.eml | Microsoft Outlook Express electronic mail |
.eps | Adobe Illustrator Encapsulated PostScript vector graphics |
.evt | Event log |
.exe | Executable program |
.f77 | Fortran 77 source code file |
.faq | Frequently Asked Questions text file |
.fax | Incoming fax |
.fd | Microsoft Fortran Declaration file |
.ff | Agfa Compugraphics Outline font description |
.ffl | Microsoft Fast Find file |
.ffo | Microsoft Fast Find file |
.ffx | Microsoft Fast Find file |
.fhx | Macromedia FreeHand version x vector graphics |
.fh4 | Macromedia FreeHand 4.x vector graphics |
.fi | Microsoft Fortran Interface file |
.fif | Fractal Image Format file |
.flc | Autodesk Animator animation |
.fli | Autodesk Animator animation |
.fm | FrameMaker document |
.fm1 | Lotus 1-2-3 release 2.x spreadsheet |
.fm3 | Lotus 1-2-3 release 3.x spreadsheet |
.fmb | WordPerfect for Windows File Manager button bar |
.fnt | Font file |
.fon | Windows 3.x font library font file |
.fot | Windows Font Installer installed TrueType font |
.frf | FontMonger Font |
.frm | Visual Basic form |
.fts | Windows Help full-text search index file |
.fxs | WinFax fax transmit format graphics |
.gadget | Windows Gadget |
.gcd | Graphics |
.gwi | Groupwise file |
.gz | GNU ZIP compressed file archive created by GZIP |
.h | Header file (used with .c source code) |
.h++ | C++ header file |
.hdf | Help Development Kit help file |
.hgl | HP Graphics Language graphics |
.hh | C++ header file |
.hlp | Help information |
.hpf | PageMaker HP LaserJet fonts |
.hpj | Microsoft Help Compiler help project |
.hqx | Compressed Macintosh file archive created by Binhex |
.htm | HyperText Markup Language document |
.html | HyperText Markup Language document |
.htt | HyperText template |
.hxx | C++ header file |
.ical | iCalendar file |
.icb | Bitmap graphics |
.icn | Icon source code file |
.ilk | Microsoft ILink incremental linker outline of program's format |
.imp | Lotus Improv spreadsheet |
.imz | Compressed floppy image |
.inc | Include file (programming) |
.inf | Information text file |
.inf | Install script/Driver information |
.ini | Initialization file |
.ion | File description (e.g., descript.ion) |
.iso | CD or DVD image file |
.isr | Microsoft Streets & Trips route file |
.jar | Java archive file |
.jas | Graphics |
.java | Java source code file |
.jff | JPEG File Interchange Format bitmap graphics |
.jpg | JPEG image file |
.jsp | Java Server Page |
.jtf | JPEG Tagged Interchange Format bitmap graphics |
.kar | MIDI file with karaoke word track |
.key | Keyboard macros |
.lbm | DeluxePaint bitmap graphics |
.ldb | Microsoft Access data |
.lex | Lexicon |
.lha | Compressed file archive created by LHA/LHARC |
.lib | Library file (programming) |
.lif | Compressed file archive |
.lnk | Windows shortcut |
.log | Logfile (text) |
.lrf | Microsoft C/C++ Linker response file |
.lwz | Microsoft Linguistically Enhanced Sound file |
.lzh | Compressed file archive created by LHA/LHARC |
.lzs | Compressed file archive created by LARC |
.lzx | Compressed file archive |
.m3u | Media playlist |
.mak | Makefile |
.mak | Visual Basic project file |
.man | Command manual |
.map | Color palette |
.map | Linker map file |
.mat | Matlab datafile |
.mbx | Eudora mailbox |
.mda | Microsoft Access data |
.mdb | Microsoft Access database |
.mdl | 3D Design Plus model |
.mdt | Microsoft ILink incremental linker data table |
.me | ASCII text file (e.g., READ.me) |
.mht | Microsoft MHTML document |
.mic | Microsoft Image Composer file |
.mif | FrameMaker Maker Interchange Format |
.mim | MIME file (used for email attachments) |
.mk | Makefile |
.mke | Microsoft Windows SDK makefile |
.mmf | Microsoft Mail mail message file |
.mny | Microsoft Money Account book |
.mov | QuickTime movie |
.mp2 | MPEG audio or video file |
.mp3 | MPEG Layer 3 audio file |
.mp4 | MPEG-4 movie |
.mpc | Microsoft Project Calendar file |
.mpe | MPEG movie clip |
.mpd | Microsoft Project database file |
.mpg | MPEG movie |
.mpp | Microsoft Project project file |
.mpv | Microsoft Project view file |
.mrb | Microsoft C/C++ Multiple Resolution Bitmap graphics |
.msc | Microsoft C makefile |
.msi | Windows Installer file |
.mso | Microsoft FrontPage file |
.msp | Microsoft Paint bitmap graphics |
.mst | Microsoft Windows SDK setup script |
.msw | Microsoft Word text file |
.mswmm | Windows Movie Maker project |
.mxt | Microsoft C data |
.nfo | Info file (ASCII) |
.nlm | Netware Loadable Module |
.nsf | Lotus Notes/Domino database |
.nt | Windows NT Startup files |
.ntf | Lotus Notes/Domino template file |
.nws | Info text file |
.nxt | NeXT format sound |
.ofm | Adobe font |
.oft | Microsoft Outlook Item template |
.ogg | Ogg Vorbis music file |
.old | Backup file (generic) |
.one | Microsoft Office OneNote file |
.ost | Microsoft Outlook Offline file |
.out | Output file (ASCII) |
.p65 | Adobe PageMaker v6.5 |
.pab | Microsoft Outlook personal address book |
.pak | Compressed file archive created by PAK; also a Quake game file |
.pal | Color palette |
.pas | Pascal source code file |
.pb | WinFax Pro phonebook |
.pbi | Microsoft Source Profiler profiler binary input |
.pbm | Portable Bit Map graphics |
.pbo | Microsoft Source Profiler profiler binary output |
.pbt | Microsoft Source Profiler profiler binary table |
.pcc | PC Paintbrush Cutout picture vector graphics |
.pcd | Kodak PhotoCD graphics |
.pcf | Microsoft Source Profiler profiler command file |
.pch | Microsoft C/C++ precompiled header |
.pcl | HP Printer Control Language HP-PCL graphics data |
.pcx | PC Paintbrush bitmap graphics |
.pda | Bitmap graphics |
.pdb | Palm database |
.pdd | Adobe PhotoDeluxe image |
.pdf | Adobe Portable Document Format |
.pfb | Type 1 font file |
.pfm | Type 1 font metric file |
.pgm | Portable Grayscale bitmap graphics |
.pgp | Pretty Good Privacy RSA System Support file |
.ph | Microsoft C/C++ Phrase-table |
.ph | Perl header file |
.pic | PC Paint bitmap graphics |
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