Windows XP Pro: The Missing Manual, Second Edition

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 document comes complete with a normally invisible filename extension (or just file extension)—a period followed by a suffix that’s usually three letters long. 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
Agenda.wpd	Corel WordPerfect
A home movie.avi	Windows Media Player
Animation.dir	Macromedia Director

Tip

For an exhaustive list of every file extension on the planet, visit http://whatis.com; 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 Tools→Folder Options from the menu bar of any folder window. When the Folder Options box appears, simply click the File Types tab (Figure 5-8).

Displaying Filename Extensions

It’s possible to live a long and happy life without knowing much about these extensions. Indeed, because file extensions don’t feel very user-friendly, Microsoft designed Windows to hide the suffixes on most icons (see Figure 5-9). If you’re new to Windows, and haven’t poked around inside the folders on your hard drive much, you may never even have seen them.

Figure 5-8. Each ...