File extensions are a very basic concept in understanding how Windows manages files. Experienced PC users may assume that it is something that everybody understands. Nonetheless, many PC users do not even know that file extensions exist and many others are confused by their function. The unfortunate decision by Microsoft to hide file extensions in the default settings for Windows has helped obscure the very important role that file extensions play in the way the Windows operating system deals with files.
I have put together a little guide in the form of a FAQ that will make clear to average PC users what file extensions are and what they do. It will fill in a common gap in understanding how Windows works and help a typical PC user make better and safer use of Windows.
1. What is a file extension?
Almost every file name in Windows has a period followed by some characters at the end. For example, a file might be called myfile.docx, where “docx” is the extension. Back in the days of DOS, extensions were limited to three or fewer characters. Current versions of Windows can have longer extensions but most file names still have fairly short extensions of 3 or 4 characters.
2. Why does a file name need an extension?
The extension is used to tell the Windows operating system what kind of file is involved. When a file is clicked or invoked in some way, Windows consults a list of extensions in the Registry to see what it is supposed to do with the file. 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. For example, information in the Registry would allow Windows to recognize that myfile.docx was a Microsoft Word file (unless you had no program installed to read or edit this type of file). If Windows can’t find the extension listed in the Registry, it will display a message indicating that it does not know what to do with the file and will ask you how you want to open the file.
3. Why don’t I see extensions on the file names displayed on my computer?
In spite of the fact that many computer professionals and educators consider it a bad setting, Microsoft has for years hidden file extensions in the default setting for Windows. Unless you change the setting, file extensions will not be shown.
4. So how do I see what the extensions are on file names?
Fortunately, the Windows default setting is easily changed so that file extensions are visible. Personally, I think everyone should change the setting so that file extensions are shown. I have described how to do this for Windows XP through Windows 8.1 at this site.
The procedure for Windows 8.x is especially simple. Open File Explorer, click or tap the tab “View”. In the ribbon, put a check in the box by “File name extensions”.
5. Why do I need to see file extensions?
Not seeing the file extension means that you may not know what kind of file you have. This can cause uncertainty about what to do with a file. A common problem that people have is when the wrong (or no) software is associated with some file extension. For example, see this article about the problems that people sometimes have with files they have received as email attachments.
Not knowing the extension is also a possible security hazard. For example, it’s important to know if a file is potentially dangerous when deciding whether an email attachment or a file downloaded from the web is safe to open. Malware sometimes masquerades as a harmless e-mail attachment.
6. How many file extensions are there?
There are many thousands and the number keeps increasing as new types of files are created. As indicated below, there are various online sites with extensive lists.
7. How can I remember so many different file extensions?
It is not necessary since there are a number of websites with compilations of known extensions. Here are several comprehensive listings:
8. What are the most common extensions?
Most people encounter only a few dozen common extensions. A table of some of the extensions usually encountered can be found here.
9. Are any file extensions potentially dangerous?
Any file type that can make changes on your computer is potentially dangerous if misused, which is why it is necessary to keep an eye on the extensions of any files that you download. A list of executable file types can be found here. A smaller list of potentially dangerous extensions is at this link.
10. What happens if I rename a file extension?
Changing the extension on a file is generally not a good idea. There can be special circumstances when this is done but usually it will result in the wrong software being used to open or access the file. Trying to open the file will then trigger an error message.
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This tips section is maintained by Vic Laurie. Vic runs several websites with Windows how-to's, guides, and tutorials, including a site for learning about Windows and the Internet and another with Windows 7 tips.
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