FileName.ext  : the delimiter is the (last) dot

FileName.EXT : the extension is EXT


The extension defines the file "type", eg text file, executable file, database file or graphic file or .... Actually a remnant from old DOS 8.3 filename limits, it has proven its usefullness over the years and today even normaly "extension-free" systems like UNIX use more and more extensions. An extension lets not only the user easily find out what sort of file it is, but even the OS (or other programs) can easily associate certain file-extensions with certain programs, thus eg starting a certain application when the user clicks on a certain (data-)file.

Thats the theory. In practice it shows that the combinations out of 3 letters are limited and thus there are ambiguities. Since there are absolutely no further investigations done by the OS what concerns the file-type, misunderstandings can happen. Indeed they happen more and more, the more identical extensions are used by different programs. Although today there is no more need to reduce the extension to 3 characters, it seems most programmers stick to it.

So even unexperienced users should be aware of the fact that a .doc file - from a friend, out of the net - needs not necessarily be a Word document, that dll's aren't limited to the dll-extension (Microsoft themselves in their ultimate wisdom breaks this rule often), that some extensions are used by ten different programs(with differing content well understood!).

So if you got a file with an unknown extension or with a well known extension that the associated program can't understand, this extension list can be a first hint what sort of file it may be. If the here listed program too can't open your file, further investigation is necessary. There are automatic file-recognition programs in the net, but their capabilities are limited. Most of them use only the extension to find out what the file actually is. Experienced users will use "dump" or "list" to find out what sort of program it is (for dump or list see soon the new converter page here ). But a lot of knowledge and skill is necessary to find out in this way what the file actually is. Further information on file formats can be found on e.g. www.wotsit.org   (very special information normaly thought only for programmers).



 Associating File Extensions under Windows

To associate a file extension with an application:

  1. In a folder or Explorer Window, click View, Options on the Menu Bar. Click on the File Types tab to display the File Types dialog.
  2. Click the New Type button to display the New File Type dialog.
  3. Type a description and a three or more letter extension in the appropriate text boxes. You do not have to type the period.
  4. Click the New button to display the New Action dialog. Type the action you want in the Action text box. To start an application, type Open.
  5. Type the application used to perform the action in the Application text box. Use the complete path to the application and add any desired command line parameters. You can use the Browse button to find the application.
  6. Click OK. Repeat this procedure if you wish to define more than one action.
  7. Select an icon for the new file type if you don't wish to use the default icon or if there is no icon defined (MS-DOS applications).
  8. Click OK to save your new file type.

Note: You cannot duplicate an extension already registered.

You can change a previously registered file type (but not the extension) by highlighting its description and clicking the Edit button. If you wish to change the extension, you must delete that extension first, then create the new extension as described above.

Associating Multiple Registered File Extensions

You can associate multiple extensions with one file type by doing the following:

  1. Create a new file type as described above.
  2. For each additional extension, perform only steps 1-3 described above and do NOT type a description. Do NOT specify an action.
  3. Click OK
  4. Open the Registry Editor.
  5. Expand the HKEY_CLASSES_ROOTS branch and scroll down the list of extensions until you find the one you entered in step 1.
  6. Highlight the extension and note the text in the right pane of the Registry Editor. This should correspond to what you typed in the Description box.
  7. Locate each additional extension you added in step 2. Highlight the extension.
  8. Double-click the word Default in the right pane. Change the text in the Value data text box in the Edit String dialog to match the text in step 5.
  9. Scroll down past the extensions to the file types. You should find the file type you described in step 1.
  10. Locate the first extension you entered in step 2. Highlight it and press the Delete key. Click OK to confirm the deletion. Repeat this procedure for any remaining extensions.
  11. Close the Registry Editor and check your work by double-clicking on a file with each extension. The desired application should run with the file opened for editing.


If you couldn't find the extension you searched:

 the list is permanently enhanced, revisit this site in a month or so.

More info on file types, Mac file types, Unix file types


Select Extension by starting letter:

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U -

V - W - X - Y - Z - OTHER - Allowed chars in Filenames

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If you find an extension missing, write! No guaratee given for completeness or correctness. If you feel an extension is outdated, write!
Authors and retailers: it is in the interest of us all and our customers to avoid the double use of extensions! Please help us keep this compilation up to date!
Send extension data to: update@file-ext.4uj.org
Compilation Copyright (c) R.Cooper-Bitsch, visit: also: www.sunorbit.net