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File Extensions

Viewing File Extensions

File extensions are the three letters that appear to the right of the “dot” in a filename. For example, document.doc or bigfile.txt. The .doc and .txt are both file extensions. If Windows is not configured to display file extensions, the filenames would appear as document or bigfile.

To configure Windows to display file extensions, follow this click sequence:

Click My Computer > View > Folder Options > View.

Make sure there is no check mark by Hide File Extensions.

Click OK when you’re done.

NOTE: To view file extensions using Windows XP, from Windows Explorer's toolbar, click Tools > Folder Options. On the View tab, remove the check mark from the "Hide Extensions for Known File Types" check box.

File Extensions

If you receive a file attachment and don’t recognize the three-letter extension (i.e. .txt or .exe), a visit to any of the following sites will shed some light on your UFO (Unidentified File Object):

File Ext contains a listing of every known file format in the world -- possibly the universe.

File Types Dictionary Scroll down through the extensive list of file extension to determine what program created the file.

Wotsit's Format contains file format information for hundreds of different file types and all sorts of other useful information. contains a searchable database of file extensions with detailed explanations of each file type. includes a comprehensive file-extensions list with easy-to-understand descriptions and associated programs.

For your file-extension pleasure, I have gathered some of the most popular file extensions you are likely to encounter in the wild and wacky world of computers and the Internet. Below is a brief description of the type of file, the program needed to open the file, and where to find it. Continue reading and you’ll find instructions for associating or registering a file extension with a particular program and how to edit or change an existing registration.

Mr. Modem Bonus Tip: If you ever have difficulty sleeping, print out this list and keep it on your nightstand. Next time you’re tossing and turning, just start reading. By the time you get to things that go .bmp in the night, you’ll be asleep.

File Extension (FE)
File Type/Description (FT)
Programs Used to Open (PO)

FE - .asc
FT - American Standard Code; an unformatted, plain text file.
FO - Any word processing program

FE - .avi
FT - Audio Video Interleave; contains audio and video
PO - Windows Media Player or Real Player. MAC: Real Player

FE - .bmp
FT - Bitmap Image
PO - Any Paint or graphics program

FE - .db or .dbf
FT - Database file
PO - Database program such as Microsoft Access

FE - .doc
FT - Microsoft Document file
PO - Microsoft Word

FE - .exe
FT - Executable file
PO - Double-click to launch an .exe file

FE - .gif
FT - Graphics Interchange Format
PO - Internet Explorer or Netscape, Paint or graphics program.

FE - .htm (.html)
FT - Hyper Text Markup Language
PO - Any browser or Web page editor, i.e. Front Page, Composer.

FE - .jpg (.jpeg)
FT - graphics file. (Joint Photographic Expert Group)
PO - Internet Explorer or Netscape, Paint or graphics program.

FE - .log
FT - Log file; usually a text file
PO - Any word processor or text editor (Windows’ NotePad or WordPad)

FE - .mdi
FT - Audio file; MIDI (Musical Instrument Digital Interface)
PO - Windows Media Player, Real Player, Quicktime; MAC: Real Player or Quicktime.

FE - .mme
FT - MIME (Multipurpose Internet Mail Extension)
PO - Most e-mail programs except AOL’s.

FE - .mov
FT - Movie file
PO - Windows Media Player, Real Player, Quicktime; MAC: Real Player or Quicktime.

FE - .mpg, mpe (.mpeg)
FT - Video file (Motion Picture Experts Group)
PO - Windows Media Player, Real Player, Quicktime; MAC: Real Player or Quicktime.

FE - .mp3
FT - Audio file
PO - PO - Windows Media Player, Real Player, Quicktime; MAC: Real Player or Quicktime.

FE - .pdf
FT - Adobe’s Portable Document File
PO - Adobe Acrobat Reader

FE - .ppt
FT - Microsoft PowerPoint file
PO - Microsoft PowerPoint

FE - .ra (.ram)
FT - Audio file; Real Audio Media file.
PO - Real Player

FE - .tif (.tiff)
FT - Graphics file; Tagged Image Format File
PO - Most graphics programs

FE - .wav
FT - Audio file; Waveform Audio file.
PO - Windows Media Player or Real Player; MAC: Real Player

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FE - .wpd
FT - WordPerfect Document
PO - Correll’s WordPerfect

FE - .xl
FT - Excel for Mac file
PO - Microsoft Excel for the MAC

FE - .xls
FT - Excel for Windows file
PO - Microsoft Excel for Windows

FE - .zip
FT - Compressed or zipped file
PO - Window users, Winzip; MAC users, ZipIt or Stuffit

Where to Fnd the Programs:

Internet Explorer
Windows Media Player - Included with Windows
NotePad - Included with Windows
Paint - Included with Windows
Adobe (for PDF files)
WordPad - Included with Windows

Associating (Registering) Programs to Open Files

When you click a file bearing a three-letter extension and a specific software program launches so you can view the selected file, that’s called an “association.” The file extension is “associated” with a particular software program used to open or display the file. Sometimes “associating” a file type is referred to as “registering” a file type. Regardless of the high-tech terminology, they mean the same thing: To use a particular software program to open a file with a specific file extension.

Now that we have all that high-falootin’ information, let’s take a look at how to associate or register a file type so it automatically launches the appropriate program needed to display its contents or perform a specific action. Before you attempt this, you must know what program you want to use to open a file. For our example, we’ll use the .doc extension and whenever our computer encounters a .doc file, we want to instruct the computer to launch Microsoft Word to open it.

Follow these steps to associate a new file type.
(Windows XP Users, click HERE for instructions.)

(Windows Visa Users, click HERE for instructions.)

1. Open Windows Explorer. (Start > Programs > Windows Explorer)

2. Select View > Folder Options.

3. Click the File Types tab which will display each of your currently registered file types and their associated applications -- which is a fancy way of saying “software programs.”

4. Select New Type which will display the Add New File Type dialog box. Start here to begin adding the new file type.

5. In the Description field, type in the type of file it is, if you know. In our example, we’ll type in “Microsoft Word documents.”

6. In the Associated extension, type in the three-letter extension of the file that you would like to open. In our example, we type in “doc.”

7. Click New to display the New Action dialog box.

8. Type the following text in the Action text box: Open with Word. (You would type in the name of the program that you’re going to use, of course.)

9. In the Application Used to Perform Action field, here’s where you’ll type in the path to the executable file -- the file that launches the program that you want to use to open the file. In our example, I would type in:
C:\Program Files\Microsoft Office\Office\winword.exe

Mr. Modem Bonus Tip (No extra charge): If you don’t know the correct path to a particular program, use the Browse button to search your hard drive for the program.

10. Click OK to close the New Action dialog box.

11. Click Close to close the Add New File Type dialog box. The Folder Options dialog box should now display your new associated (registered) file type.

12. Click OK to close the Folder Options dialog box.

Using our example, the next time somebody sends me a .doc file as an attachment, I could either scream, “What’s up, .doc?” or just double-click the filename, which would cause Microsoft Word to launch. Once launched, it would automatically display the contents of the .doc file. Voila!

Editing an Existing File Type Registration

There may be occasions when you’ll want to change the action associated with a previously registered file. For example, if you upgrade to a new version or you purchase a new computer, you might need to make an adjustment. Follow these simple steps to edit an existing file type:

1. Open Windows Explorer. (Start > Programs > Windows Explorer)

2. Click View > Folder Options.

3. Select the File Types tab. (Try saying that fast three times in a row!)

4. Select the file type you want to edit or change.

5. Click Edit to display the Edit File Type dialog box.

6. To modify the current action, click Edit, change the entry in the Application Used to Perform Action text box, then click OK.

7. To add an action, click New and complete the Action text box and the Application used to execute the action text box. Then click OK.

8. Click OK twice to close the dialog boxes.

To change File Associations in Windows XP:

1. Open Explorer and choose Tools > Folder Options.

2. Click File Types tab.

3. Scroll to and click the file type you want to change.

4. Click the Change button. The "Open With" dialog box will appear.

5. Click the name of the program that you want to open that particular file type.

6. Click OK to save your settings and close the dialog box, then OK again to close the Folder Options dialog box.


1. Right-click a file with the extension whose association you want to change, and then click Open With.

2. In the Open With dialog box, click to select the program you want to use to open the file, or click Browse to locate the program you want to use.

3. Select the "Always use the selected program to open this kind of file" check box.

Quick Change Artist

Many associations were provided when Windows was installed on your computer. There may come a time, however, when you may want to change the default association and use another program to open a particular file type. The easiest way to accomplish this is to follow these steps:

1. Locate the file association you would like to change. I prefer to use Windows Explorer (Start > Programs >Windows Explorer), but any method that you’re most comfortable using is the best one to use.

2. Click on the filename once to select it, then hold down the Shift key while you right-click on it. Select Open With from the menu that appears.

3. Select the desired program, then check the "Always use this program
to open this type of file" box and click on OK.

Another quasi-helpful Mr. Modem Bonus Tip: If you don't find the program you’re looking for in the list, click the Other button and navigate to the correct program on your drive.

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