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File Extensions and Associations (modified 7-5-16)

See also this page for further discussion on changing associations

Naming files in Windows includes a file name and a file extension. The file extension is not normally visible in a default Windows installation. In old DOS and Windows 3.1, the rules for naming files was very restrictive.

Since Windows 95, file names can be almost anything including upper case, spaces, some punctuation and up to 255 characters.

File extensions have remained fairly consistent being normally limited to 3 characters. The difference is that in the old systems, we created our own file extensions to suit our needs. Since windows 95, the programs assign the extension and if we try to override it, we make a mess.

2016 update: File extensions can be almost anything within the DOS rules: no special characters like the / : - etc.

If you want to turn on the feature to make file extensions visible, open My Computer or Windows Explorer, Click on the menus and look for options or Folder Options. Each version of Windows has changed the location and name. You should get a dialog box that is titled Folder Options. Click the View tab and find the box that says "Hide File Extensions For Known File Types" and click the box to remove the check. If it is already unchecked, leave it.

In Windows 10 the menu at ther top of the Explorer window has a selection called View. Selecting that shows some boxes. If checked the item is selected. If blank you need to check it.

The danger in having the extensions visible is this: If you change a file name, you must also include the correct extension. If you leave the option to hide extensions, then the extension is automatically added to a changed file name. Again since about windows 7, when you choose to rename a file, the extension does not change. BUT CAUTION: If you highlight the entire file name and extension, it can be lost.

What is the purpose of an extension? Windows has a registered association between the file extension and the program it is associated with. For example, a .doc file is associated with either WordPad or Word. A .txt is associated with Notepad. If you ask the computer to open a file with a known extension (By double clicking it), Windows will automatically open the correct program and open the file in that program.

You can view the file associations currently in use by opening the folder options discussed above and clicking on the File Types tab. You will see a list programs and their associated extensions. Windows Me has changed the appearance of the dialog box, but the information is still there.

These associations can be changed. The easiest way to do it is to find a file with the extension to be associated with a different program, and Right Click on the file name. Choose "Open With" from the menu. Scan through the list of programs in the Open With dialog box and select the one you want. If you want the change permanent, also check the "Always use this program..."

The biggest problem we have today is graphics files. Every time you install a new graphics program, the new program assumes control over all the graphic extensions. Then if you decide to uninstall it, the extensions are orphaned and windows does not know how to open them. You then get a dialog box that says "Open With" and shows a list of programs. Just choose a program from the list. Be sure it is one that will handle the file type.

For detailed instructions on changing file associations, Click Here.

.2GR APPLIED TO Win 3x on 286 or 386 computers
.3GR Special Windows function on 386, 486 and Pentiums
.386 Swap files and others that help run in enhanced mode
.ANI Animated Cursors
.ARC Compressed file format
.AVI Movie file
.BAK Backup file
.BAT Batch file. Runs a series of DOS commands
.BMP Bit Map picture file. The only format used for wallpaper. Can view in Paint.
.CAB Cabinet file. Compressed files holding program setup files.
.CAL Calendar file
.CBT Tutorial file
.CFG configuration file usually used with MIDI music
.CHK Bad data converted to a file by the Checkdisk or Scandisk program.
.CLP files created by Windows Clipboard
.COM DOS/Windows system file
.CPL Control Panel file
.CRD Windows Cardfile file (Old program from Win 3x)
.CUR Cursor files
.DAT Data files
.DEV Device Driver File
.DLL Dynamic Link Library file used by most Windows programs to activate special instructions
.DOC Document files created by Microsoft WordPad and Word and old versions of WordPerfect
.DRV Device Driver File
.EPS Postscript Printer File
.EXE Main file that runs a Program
.FON Font file used in Win 3x
.FOT Font file used with Win 3x
.GIF Picture/Graphic file
.GRP Group file for menus
.HLP Help files
.HST History files
.ICO Icon files
.INF Information files associated with a Program.

.INI Initialization files contain information to set options for a program. Most common are the Win.ini and System.ini in Windows. Run Sysedit.exe to look at these.

.JPG and .JPEG Picture/Graphics file. Used extensively on the Web and in Digital cameras. Cannot view with older versions of Paint. Newer versions work with .jpg files. Typically in Win Me, 2000 & XP
.MID and .MIDI Sound files
.MPG Movie files
.MSP Old graphics format
.NET Network files
.OVR Overlay files (small parts of larger files)
.PAL Palette file for use with Paint programs
.PAR Permanent Swap File
.PCX Another Graphic format
.PDF Adobe Acrobat file. Needs Acrobat Reader to open it.
.PIF Program Information File for Windows to run a DOS program
.PIX Picture/Graphic format
.REC Windows Recorder files
.RLE Picture/Graphic files
.RTF Rich Text Format (Similar to .TXT but with better looking text. Can be read by any Wordprocessor.
.SCR Screen Saver Files
.SRC Files used to create .INI files
.SWP Temporary Swap files. Space on Hard drive for Windows to hold data when the RAM is full.
.SYD Backup file created by Sysedit
.SYS System files
.TIF/TIFF Picture files
.TMP Temporary files created while programs are in use. Should be deleted when the program closes properly.
.TRM Windows Terminal files
.TTF True Type Font file (the most common on in Windows)
.TXT Text files created and readable by any wordprocessor
.VOC Sound files
.WAV Sound files (Most common in Windows)
.WIN Windows Backup files
.WPD Document files created by WordPerfect
.WRI Old files created by Win 3x Write program (replaced by WordPad in Win 9x. WordPad will read .WRI files
.ZIP Compressed files. Cannot be used until unzipped with PKZip or WinZip

For a discussion of what files can be deleted to cleanup your hard drive, click here

See this page for additional discussion of e-mail virus problems.

Some well known viruses transmit themselves in a form that appears to be a picture file, like OPENME.JPEG.vbs. People often don't see the final .vbs and assume the attachment is a safe picture. It's not. According to Microsoft, files ending with the extensions shown in the list below can contain executable code, and contain a virus.

.ade Microsoft Access project
.adp MS Access project
.bas MS Visual Basic class module
.bat Batch file
.chm Compiled HTML Help file
.cmd Windows NT Command script
.com MS-DOS program
.cpl Control Panel extension
.crt Security certificate
.exe Program
.hlp Help file
.hta HTML program
.inf Setup Information
.ins Internet Naming Service
.isp Internet Communication settings
.js JScript file
.jse Jscript Encoded Script file
.lnk Shortcut
.mdb MS Access program
.mde MS Access MDE database
.msc MS Common Console document
.msi Windows Installer package
.msp Windows Installer patch
.mst MS Visual Test source files
.pcd Photo CD image, Visual compiled script
.pif Shortcut to MS-DOS program
.reg Registration entries

The pages below reflect a quick search on Google for sites with extensive lists of File Extensions. Still valid in 2016

Click here to check Whatis

Click here to check

Click here to check a New Zealand site

Click here to check another source

And finally what is a very complete list at the Tech Dictionary

Note: this site is provided free. Donations are accepted to help support the work. Click here for instructions.

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