Can't unhide filename extensions
After doing some research I found the answer.  You have to run this command:

SetFile -a e &lt;filename&gt;


In effect, this says "Set the file extension attribute (-a e) so that the extension is not hidden.  If the extension were not hidden and you wanted to hide it, you would substitute "E" for "e".

To see all of a file's attributes, run this command:

GetFileInfo &lt;filename&gt;


A description of all the attributes can be found here.