Common Filename Extensions

Appendix. Common Filename Extensions

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:{your extension}

    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 ( 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




Temporary file


ASCII text file (e.g., READ.1st)


3D Studio file

.3gp, .3gpp

Video clip created on a cell phone or other mobile device


Windows Video Grabber datafile


PageMaker Printer font with Math 8 extended character set


PageMaker Printer font with Roman 8 extended character set


Adobe Binary Font


Photo album


MPEG audio sound file


Microsoft Agent Character file


Ace Archiver compressed file


Microsoft Agent Character file


AfterDark screensaver


AutoCAD graphics


Archetype designer document


Truevision bitmap graphics


ATM Type 1 font metric ASCII data for font installer


Adobe Illustrator vector graphics


ANSI graphics; character animation


Microsoft Visual C++ file


Compressed file archive


Automatic Response File


Compressed file archive created by ARJ


ASCII text file


Microsoft ASP.NET user control file


Word for Windows Autosave file


Assembly source code file


Microsoft .NET Web Service file


Microsoft Active Server Page


Microsoft ASP.NET file


Microsoft Windows Media Active Stream Redirector file


Windows Media Streaming video shortcut


Adobe Type Manager data/information


Audio file used on older web pages


Video for Windows Audio Video Interleaved movie clip


Animation Works movie


3D Builder file


Backup file (generic)


Basic source code file


DOS batch file


Bitstream Outline font description


Microsoft Publisher Border


Bitstream Outline font description


Bitmap graphics


Bibliography (ASCII)


Binary file


Bitmap X11


Microsoft backup file


FontEdit mirror image of font set


Bitmap graphics


Corel image file


Windows bitmap


Microsoft Fortran Pwbrmake object file


Backup file (generic)


C source code file


C++ source code file


Microsoft installation archive Cabinet file


Comma-delimited ASCII file


C++ source code file


Visual Basic animated button configuration


Autodesk AnimatorLumena Graphics


Configuration file


Common Gateway Interface script


Computer Graphics Metafile vector graphics


FontChameleon Font descriptor


ChkDsk recovered data


Caltech Intermediate Format graphics


Easy CD Creator CD image file


Compiled Java class file


C++ class definition file


Creative Music File FM-music file


Helpfile contents


Microsoft Multiplan spreadsheet


Windows Contacts contact entry


Colorlab Processed Image bitmap graphics


DOS Code Page Information file


C++ source code file


Colored PostScript files


Microsoft MASMZortech C++ cross-reference


C# source code file


Comma Separated Value text file format (ASCII)


Microsoft online guides Course TeXt file


Pretty Good Privacy RSA System cipher text file


WinFax cover page


Canvas graphics


Datafile (generic)


Microsoft C/C++ symbolic debugging information


Shockwave file


CYMK format bitmap graphics


Bitmap graphics


Device driver (old)


Device-Independent Bitmap bitmap graphics


Lotus Notes/Domino dictionary file




Description In Zip description file


Microsoft Windows SDK dialog resource script file


Microsoft Word document


Word for Windows template


Device driver (old)


Windows Media Center recorded video file


AutoCAD Drawing Interchange File format vector graphics


Enhanced Metafile graphics


Microsoft Outlook Express electronic mail


Adobe Illustrator Encapsulated PostScript vector graphics


Event log


Executable program


Fortran 77 source code file


Frequently Asked Questions text file


Incoming fax


Microsoft Fortran Declaration file


Agfa Compugraphics Outline font description


Microsoft Fast Find file


Microsoft Fast Find file


Microsoft Fast Find file


Macromedia FreeHand version x vector graphics


Macromedia FreeHand 4.x vector graphics


Microsoft Fortran Interface file


Fractal Image Format file


Autodesk Animator animation


Autodesk Animator animation


FrameMaker document


Lotus 1-2-3 release 2.x spreadsheet


Lotus 1-2-3 release 3.x spreadsheet


WordPerfect for Windows File Manager button bar


Font file


Windows 3.x font library font file


Windows Font Installer installed TrueType font


FontMonger Font


Visual Basic form


Windows Help full-text search index file


WinFax fax transmit format graphics


Windows Gadget




Groupwise file


GNU ZIP compressed file archive created by GZIP


Header file (used with .c source code)


C++ header file


Help Development Kit help file


HP Graphics Language graphics


C++ header file


Help information


PageMaker HP LaserJet fonts


Microsoft Help Compiler help project


Compressed Macintosh file archive created by Binhex


HyperText Markup Language document


HyperText Markup Language document


HyperText template


C++ header file


iCalendar file


Bitmap graphics


Icon source code file


Microsoft ILink incremental linker outline of program's format


Lotus Improv spreadsheet


Compressed floppy image


Include file (programming)


Information text file


Install script/Driver information


Initialization file


File description (e.g., descript.ion)


CD or DVD image file


Microsoft Streets & Trips route file


Java archive file




Java source code file


JPEG File Interchange Format bitmap graphics


JPEG image file


Java Server Page


JPEG Tagged Interchange Format bitmap graphics


MIDI file with karaoke word track


Keyboard macros


DeluxePaint bitmap graphics


Microsoft Access data




Compressed file archive created by LHA/LHARC


Library file (programming)


Compressed file archive


Windows shortcut


Logfile (text)


Microsoft C/C++ Linker response file


Microsoft Linguistically Enhanced Sound file


Compressed file archive created by LHA/LHARC


Compressed file archive created by LARC


Compressed file archive


Media playlist




Visual Basic project file


Command manual


Color palette


Linker map file


Matlab datafile


Eudora mailbox


Microsoft Access data


Microsoft Access database


3D Design Plus model


Microsoft ILink incremental linker data table


ASCII text file (e.g.,


Microsoft MHTML document


Microsoft Image Composer file


FrameMaker Maker Interchange Format


MIME file (used for email attachments)




Microsoft Windows SDK makefile


Microsoft Mail mail message file


Microsoft Money Account book


QuickTime movie


MPEG audio or video file


MPEG Layer 3 audio file


MPEG-4 movie


Microsoft Project Calendar file


MPEG movie clip


Microsoft Project database file


MPEG movie


Microsoft Project project file


Microsoft Project view file


Microsoft C/C++ Multiple Resolution Bitmap graphics


Microsoft C makefile


Windows Installer file


Microsoft FrontPage file


Microsoft Paint bitmap graphics


Microsoft Windows SDK setup script


Microsoft Word text file


Windows Movie Maker project


Microsoft C data


Info file (ASCII)


Netware Loadable Module


Lotus Notes/Domino database


Windows NT Startup files


Lotus Notes/Domino template file


Info text file


NeXT format sound


Adobe font


Microsoft Outlook Item template


Ogg Vorbis music file


Backup file (generic)


Microsoft Office OneNote file


Microsoft Outlook Offline file


Output file (ASCII)


Adobe PageMaker v6.5


Microsoft Outlook personal address book


Compressed file archive created by PAK; also a Quake game file


Color palette


Pascal source code file


WinFax Pro phonebook


Microsoft Source Profiler profiler binary input


Portable Bit Map graphics


Microsoft Source Profiler profiler binary output


Microsoft Source Profiler profiler binary table


PC Paintbrush Cutout picture vector graphics


Kodak PhotoCD graphics


Microsoft Source Profiler profiler command file


Microsoft C/C++ precompiled header


HP Printer Control Language HP-PCL graphics data


PC Paintbrush bitmap graphics


Bitmap graphics


Palm database


Adobe PhotoDeluxe image


Adobe Portable Document Format


Type 1 font file


Type 1 font metric file


Portable Grayscale bitmap graphics


Pretty Good Privacy RSA System Support file


Microsoft C/C++ Phrase-table


Perl header file


PC Paint bitmap graphics

Part II: Nutshell Reference