Windows Vista in a Nutshell

Appendix D. Common Filename Extensions

This appendix lists many of the most common filename extensions that you’ll find on your system, that you might download, or that you have received over the Internet.

Extensions were universally used on DOS and Windows 3.1 files, but Microsoft has gone to some difficulty to hide them in Windows Vista. This is unfortunate, because they play a major role in the way Windows decides what application will be used to open a file, as well as which files will be visible when opening files in a given application. Although direct associations are made between some files without extensions and the applications needed to open them, in most cases the association is between an extension and a Registry setting that tells the system what application to use. To enable the display of filename extensions, open Windows Explorer, go to Control Panel → [Appearance and Personalization] → Folder Options → View tab, and turn off the “Hide extensions for known file types” option.

If you double-click on an unknown file type, the Open With dialog box appears, allowing you to make a new association. To subsequently change an association once it has been made, right-click on the file and select Open With, or select Properties and then click Change.

Table D-1 list many system extensions, but third-party applications use literally thousands of file formats. And you might be thrown off by an improperly named file, or by an application using a standard filename extension for ...