All computer users should make sure that the files they save have three letter extensions at the end of their file names. Computers use these extensions to understand which application created the file and which application(s) will open the file. Examples are listed below.

On a Windows PC the computer adds these three letter extensions automatically when saving files and most users have these extensions hidden from view so they don't see them but the computer does add them anyway.

On a Mac the computer hasn't needed any extensions to figure out what application to use to open files. At least until OSX.

Now, in OSX all files you save and/or want to open on your Mac should include one of the three letter extensions listed below, depending on the application you are using to create the file.

If you are creating the file and saving it in OSX, the application you are using should already by default want to add the appropriate extension. Problem solved.

If you are trying to open a file in OSX that someone has shared with you or you have created in the past and there is no extension on the end, you may see see a message to the effect that Your computer does not know what application to open the file with.

If you add the appropriate extension based upon the program you created the file with, its icon should change to the appropriate icon and now should open just by double clicking on the icon.

If someone sent you an attachment through email but did not include the appropriate extension in the file name, chances are you will not be able to open it at all. Even if you save the attachment and add the appropriate extension, it might tell you that the file is corrupted or something similar. Ask the person to add the extension and resend the attachment and the new attachment should now work for you.

In this world of internet, email, and multiple computer platforms, "Playing Nicely With Others" means that we will include file extensions on all files we save, making life easier for ourselves and others.

File Extension Examples