Generally, changing the file extension of a file is not a good idea.

When you double-click a file that has the right extension, it will open with the correct program. For example, a file with a .txt file will open in a text editor, and a file with a .jpg extension will open in an image viewer.

If you change a file's extension, it may cause the file not to open. Take, for example, the JPEG image below:

While the file used in the example is from a Macintosh computer, the same principles apply to both Mac and Windows platforms.

The filename of this file is "rose.jpg" and is formatted as a JPEG image. Let's see what happens if we try to change the file extension to ".gif," which is the extension GIF images use. By clicking the filename twice (slower than a double-click), we can change the filename. We replace ".jpg" with ".gif" and hit Enter.

When this change is made in Mac OS X, an alert pops up asking us to verify that we want to make the change.

This alert is for good reason, as it warns the user that the file may open in a different application if the extension is changed. The truth is, the file may not open at all. To be on the safe side, Mac OS X selects "Keep .jpg" as the default option.

But we decide to make the change and click the "Use .gif" option. The filename is changed as seen below:

The rose.jpg file opened fine in Photoshop before we changed the file extension. Now that we changed the extension let's see what happens when we drag it to Photoshop.

Photoshop tries to open the file, but is unable to. Instead, we get an error message.

Because Photoshop was expecting a GIF file type, based on the file extension, it tried to parse a GIF file. However, the file is actually formatted as a JPEG file, so PhotoShop registers an error when attempting to open it.

When you change a file's extension, you change the way programs on your computer read the file. The problem is, changing the file extension does not change the file type. In other words, when you change a file's extension, it does not change the way the file is formatted.

By modifying a file extension of a certain file, you may cause programs on your computer to read the file incorrectly, which may prevent it from opening. This is why it is typically not a good idea to change file extensions.

Sometimes, changing a file extension may be a good idea.

While file extensions should typically be left alone, there may be times when changing a file's extension is the only way to get it to open. For example, you may receive an e-mail attachment called "report.pdf" that is actually formatted as a Word document.

When you double-click the file, Acrobat Reader will most likely try to open the file, since Acrobat Reader is what most computers use to view PDFs. Since Acrobat Reader is expecting a PDF, when it encounters a Word document, it may not be able to open the file. If you change the filename to "report.doc," the file should open in Microsoft Word. Since the file type is a Word document, the file should open without a problem.

Changing a file's extension can "force" the file to open in a different program. This can be useful when a file will not open using the default program. However, a better way to change what program opens a certain file is to modify the file's properties.

See Changing What Program Opens a File in Windows and Changing What Program Opens a File on a Mac for more information.