File extensions are the part of a file name that tells the operating system what kind of file it is. The operating system uses the information to determine which software application to use to open the file, such as a word processing or spreadsheet application. Generally, files are assigned the correct extension when they are first created. However, a file may sometimes be assigned an incorrect file extension. In that case, you can change file extensions on a Mac to correct the problem.
Locate the icon for the file whose extension you want to change using the Finder's toolbar, Finder's find command or Spotlight search utility, according to your preference.
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Click the file name beneath the icon twice, more slowly than you would click for a double-click. This opens a dialog in which you can edit the file's name.
Type over the current file extension and replace it with the new extension. For example, you might rename sample.jpg to sample.gif. Press "Enter." This opens a confirmation warning prompt.
Click the button in the window that shows the new extension. In this example, the confirmation window would prompt you with two buttons, a "Use .gif" button and a "Keep .jpg" button. Clicking the "Use .gif" button applies the change and completes the process.
Changing a file's extension may cause the file to become unusable by the operating system.