You do not need to add it depending on how you rename.
Open the folder, click on the Views icon and select Details to display file
names.
Do you see .jpg added at the end of each file name?
If not, click on Tools on the top bar and then Folder options and then the
View Tab.
Scroll down and remove the check in the box "Hide Extension ...." and click
OK.
You will now see the .jpg added to your file name.
Now suppose you have
Filename(with the .jpg extension hidden)
and
Filename.jpg
You rename Filename to Filename1. This will work fine because you did not
remove or change the .jpg extension, it was simply hidden from you. But if
you now add .jpg you will see the following:
Filename.jpg.jpg
if you list the files without adding the extension.
The first .jpg is the one you added and the second one was hidden. But this
will still work fine.
If however you try renaming when you see the extension, as in
Filename.jpg
and you rename it
Filename1 without adding .jpg you will get a warning message and you have to
go back and add .jpg
You can display files with or without extensions using the above procedure,
I prefer with extensions, so I know the types of files I am working with. I
have no idea why XP made the default being hiding file extensions. No longer
such in Vista.