Filename Extensions

Every operating system needs a mechanism to associate documents with the applications that created them. When you double-click a Microsoft Word document icon, for example, Word launches and opens the document.

In Windows, every file name has a suffix called a filename extension, which is a period followed by three letters. Every time you install a new program, the installer lets Windows know about the file types (the filename extensions) the new application is capable of opening. This information is stored in the Windows 2000 Registry (see page 405).

Here are some common examples:

When you double-click this icon

… this program opens it

Fishing trip.doc

Microsoft Word

Quarterly results.xls

Microsoft Excel

Home Page.htm

Internet Explorer


Corel WordPerfect

A Home Movie.avi

Windows Media Player


Macromedia Director


For an exhaustive list of every file extension on the planet, visit; click the link for Every File Format in the World.

Behind the scenes, Windows maintains a massive table that lists every extension and the program that "owns" it. To see this list, choose ToolsFolder Options from the menu bar of any folder window. As shown in Figure 7-6, the Folder Options box appears; click the File Types tab.

Every software program you install must register the file types it uses. The link between the file type and the program is called an association. This dialog box displays the icon for each file type, and an explanation of the selected listing.

Figure 7-6.  Every software program you install must register the file types it uses. The link between ...

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