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What's in a File Name? 3 Letters Say Plenty

February 08, 2001|JEFF LEVY |

When we talk about file types, we talk about files containing data in a specific format for use by a particular program or programs. Windows Explorer lists file types in its Type column.

For example, a file ending with the three-letter extension of .ZIP would be a ZIP, or compressed, file. The ZIP file type would have to be used by such programs as WinZip or PKZIP for Windows, or any other program that recognizes .ZIP files.

File extensions usually are expressed with three characters that follow the last period in the file name. Although you can change the file extension by renaming the file, the change does not alter the file type. For example, a Microsoft Word document ends with the extension .DOC. Changing that extension to .ZIP doesn't change the file type, so that file is still a Word document file.

If we reverse our example and change a .ZIP file to a file that ends with a .DOC extension, Windows will show that file as a Microsoft Word file. That's because Windows doesn't look into a file to determine the file type.

Windows relies on the file name extension to decide what program to use. As far as Windows is concerned, any file with a .DOC extension is a Microsoft Word file.

The effect of changing the extension is that Windows will attempt to open the file in an incorrect program and most likely will result in a screen full of gibberish.

Windows maintains a database or list of file extensions and the programs that use them. When Windows is instructed to work with a file, it finds the extension in its list and uses the program associated with that file type. For example, if you ask Windows to open a text file (.TXT), Windows will use Notepad to open that file.

You can look at the file type list by right-clicking on My Computer and then left-clicking on Explore. Click on View and then on Folder Options. You also can get to this area by opening Explorer as above and clicking on Tools and then Folder Options. Click on the File Types tab to display the list of file types. You can highlight any file on the list to learn its file extension. This is the method you would use for Windows ME.

There are buttons here that allow you to remove or add a new association. You also can edit a current file association and change the default program to open a particular file type. Highlight the file type in question and then click the Edit button. Click and highlight "open" and then click the Edit button. You can now change the default program to be used when opening a file with any particular file extension.

Sometimes Windows Explorer will hide file associations that deal with common file types such as .ZIP, .WAV (sound) and .JPG (graphics). When this happens, you can open Explorer as above, click on View and then on Folder Options. Click on the View tab and uncheck the "Hide file extensions for known file types" box.

There are certain files that have no associations with programs. Windows system files and support files installed by your application programs will not have associations. The programs that use these files get them on an as-needed basis.


Jeff Levy hosts the "On Computers" radio talk show from 9 a.m. to noon Sundays on KFI-AM (640).



.GIF Graphics file

.ARC ARC compressed archive file

.HTML Hypertext markup language document

.BAK Backup file

.ICO Icon file

.BAT MS-DOS batch file

.INF Information text file

.INI Initialization file (contains Windows settings)

.JPG Graphics file format

.LOG Log files containing information in a text format

.COM Executable program file

.TIF Tagged Image File (graphics file)

.TTF True Type Font

.TXT Text file

.DOC Usually MS Word document file

.DOT Template file for MS Word

.ZIP Compressed file containing larger file(s)

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