What Are Extensions and What Do They Do?

You will recall that in the Windows operating system, most files have names with the format filename.ext where .ext is the extension. An extension is not absolutely required but most files have one in their name. Extensions are often, but not necessarily, three characters. Their function is to tell the computer what the file type is and what is to be done with the file when it is opened or double-clicked or otherwise invoked one way or the other. When the operating system is presented with a file for action (e.g., when you double-click it), the Registry is consulted where a list of file types and their extensions is kept. The list contains what actions are possible for that particular file, which software is supposed to carry out the action, and where on the computer that software is located (the path).

By being aware of the role of an extension, a user can make intelligent decisions about how (or whether) to use a file. For example, e-mail viruses are usually spread by means of attachments with one of the executable extensions such as .exe, .vbs or .scr. An informed user will be able to recognize the danger in indiscriminately opening such attachments and will be prepared to take appropriate action.

Making file extensions visible

Unless you change the default system settings, Windows will not show the extensions of filenames in any listing of files. Presumably Microsoft thinks users need to be protected from themselves in this way, in spite of the fact that a lot of people think it is a poor idea for the default setting. Be that as it may, the setting is easily changed. For Windows XP, open My Computer and click on the Tools menu. Then open Folder Options|View. In the list of settings that appears, uncheck the box by the entry, "Hide extensions for known file types." For a more detailed description with graphics, go to How to Make File Extensions Visible in Windows XP. To see a short movie illustrating the process for Windows 98 click Video Clip. (Requires Flash 6 or higher. Allow time for the download.)

If the rather easy procedure for making file extensions visible given above is still too much for you, I have written a little file that makes it even simpler. Download this little INF file, put it anywhere convenient, unzip it, and right-click it. In the context menu that appears, select “Install”. That does it. You have to refresh the view (press the function key F5) but now your file extensions will be visible. In case somebody changes their mind, this file will hide the extensions again. Remember to refresh the view by pressing F5.

Next: A table of common extensions.