Most files in Windows and Macs have extensions. These three characters after the dot in a file name help the operating system identify the type of data in the file and use the correct program to open it. Mac computers hide file extensions by default, showing only the main part of the file name and an icon identifying its type. To change this, configure Mac OS X to always display file extensions, or view the information for an individual file to see and change the extension.
Showing All File Extensions
Open the Finder pull-down menu and click Preferences.
Click the Advanced tab.
Click "Show all file extensions."
Close the window. The Mac will display the three-character extension for all files. Repeat these steps and click "Show all file extensions" again to hide file extensions.
Viewing or Changing One File Extension
Right-click a file and select Get Info. If you have a Mac with a one-button mouse, hold the "Control" key and click the file.
Click the triangle next to Name & Extension. The text field will display the full file name and extension.
Click inside the text field and use the "Delete" key to remove the original file extension.
Type the desired new file extension and close the window.
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