How can I add ".mp4" to the end of multiple files in a folder?

I just received a bunch of videos (for editing) and something weird happened. I was told the files are .mp4, but when I received them they did not have a filetype assigned to them.

I can add ".mp4" to the file name, which fixes the issue and allows my applications to recognize the files. Problem is, there are a lot of files.

Is there any way to easily add a ".mp4" to the end of all of the files at once? I'm willing to download an app for this if needed (Mac OS X 10.8.5)

Thank you!