Windows 8.1: The Missing Manual

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.

Here are some common examples:

When you double-click this icon…	…this program opens it.
Fishing trip.docx	Microsoft Word
Quarterly results.xlsx	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 in the world, visit www.whatis.com; click the link for “Every file extension in the world.”

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

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 10-10). If you’re new to Windows, you may never have even seen them.

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 ...