File extensions are normally hidden on macOS. Ordinarily, you don’t need to know the extension of a file however, it’s pretty easy to show file extensions on macOS. In fact, you can choose to show extensions for all files, or you can show them for just select files.
Show file extensions
On the Finder preferences window, go to the Advanced tab. The very first option is ‘Show all filename extensions’. Select it and you will see file extensions for all files in Finder and on your desktop.
If you need to show the extension for select files, right-click the file and select Get Info from the context menu. In the window that opens, expand the Name & Extension group. Under it, you will find an option called ‘Hide Extension’, and by default, it’s enabled. Uncheck it and the extension for that file will start showing up in both Finder and on your desktop.
This option works both ways. If you’ve enabled all extensions to be shown, you can use this option to selectively hide extensions for certain files.
The neat thing about macOS is that no matter how long a file’s name is, the extension will still be visible. The name will be cut off after it reaches a certain character count but the extension is added at the end nevertheless.
Unfortunately, you cannot apply this to an entire folder. The extension option, while available in the Get Info window of a folder, is inactive. If you want to show file extensions for multiple files, you’re going to have to go about it one by one. Selecting multiple files, right-clicking one file, and selecting the Get Info option will open a Get Info window for every single file that you selected so, don’t try that unless you want to end up closing lots of windows.
Once you set file extensions to show, you can select the extension of a file much like you can its name, and change it. Be warned though that this won’t convert a file from one format to the other so unless you’re correcting a file extension, you shouldn’t edit it from a file’s name. When you do change the file extension from a file’s name, you get a confirmation message asking if you really want to change it.