What is a file extension?

In Windows, a file extension tells your computer what type of application to use to open a file. The file extension is a three-letter code that appears at the end of a file name, although it is sometimes hidden (more about that in a minute).

For example, if a file is named mythesis.doc, the .doc is the file extension, and it tells Windows that the file was created with Microsoft Word. If a file is called mythesis.xls, then the extension is .xls and this tells Microsoft Windows that the file was created with Excel.

So, where are these file extensions?

Windows will hide all file extensions by default. The idea is that most people won’t want to bother with extensions, since changing a file extension can cause trouble (if you change it to the wrong thing) and since Windows mostly takes care of dealing with extensions behind the scenes. However, there may be times when you need to see file extensions, and you can do so using the instructions below.

How do I see file extensions?

Windows 7

  • Go to Start -> Help and Support
  • Search for “Show or hide file name extensions
  • In the winow that appears, remove the checkmark from “Hide extensions for known file types”
  • Click OK

Windows Vista

  • Control Panel->Appearance and Personalization->Folder Options.
  • Click the View tab and then, look under Advanced settings
  • Look for “Hide file extensions for known file types” and remove any checkmark next to this option.
  • Click OK

Windows XP, 2000, and Me:

  • Open My Computer. Go to the Tools menu and select Folder Options.
  • Click on the View tab and scroll down to “Hide file extensions for known file types“.
  • Remove any checkmark next to this option.
  • Click OK.

Windows 98:

  • Open My Computer. Go to the View menu and select Folder Options.
  • Click on the View tab and scroll down to “Hide file extensions for known file types“.
  • Remove any checkmark next to this option.
  • Click OK.
  • Close the My Computer window.

Renaming a file

Now that you can see the full file name, it is possible to rename any part of the file, including the file extension. Be very careful and don’t do this unless you have a good reason to. You may want to make a note of the original file extension in case you need to change it back.

  1. Right-click on the file.
  2. Click Rename and make your change, then press Enter.

Example: to change mythesis.xls to mythesis.xlsREMOVE, right-click on mythesis.xls, click Rename and type in .xlsREMOVE (the dot is important) and press Enter.