Section 6.9. Filename Extensions and File Associations

6.9. Filename Extensions and File Associations

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.

Table 6-1 shows some common examples.

Table 6-1. Filename Extensions

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
Sudoku .gadget	Sidebar gadget
Animation .dir	Macromedia Director

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Tip: For an exhaustive list of every file extension on the planet, visit www.whatis.com; click the link for "Every File Format in the World."

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Behind the scenes, Windows maintains a massive table that lists every extension and the program that "owns" it. More on this in a moment.

6.9.1. Displaying Filename Extensions

It's possible to live a long and happy life without knowing much about these extensions. Because file extensions don't feel very user -friendly, Microsoft designed Windows to hide the suffixes on most icons (Figure 6-7). If you're new to Windows, you may never have even seen them.

Figure 6-7. As a rule, Windows shows filename extensions only on files whose extensions it doesn't recognize. The JPEG graphics at left, for example, don't show their suffixes. Right: You can ask Windows to display all extensions, all the time.

Some people appreciate the way Windows hides the extensions, because the screen becomes less cluttered and less technical-looking. Others make a good argument for the Windows 3.1 days, when every icon appeared with its suffix.

For example, in a single Explorer window, suppose one day you discover that three icons all seem to have exactly the same name : PieThrower. Only by making filename extensions appear would you discover the answer to the mystery: that one of them is called PieThrower.ini, another is an Internet-based software updater called PieThrower.upd, and the third is the actual PieThrower program, PieThrower.exe.

If you'd rather have Windows reveal the file suffixes on all icons, open an Explorer window. Choose Organize Folder and Search Options. In the Folder Options dialog box, click the View tab. Turn off "Hide extensions for known file types," and then click OK.

Now the filename extensions for all icons appear (Figure 6-7).