iTunes uses QuickTime for playback. Default extension to "force" iTunes to be used is .m4v.
You can right click on the file icon and choose between playback options. iTunes supports "Airplay" (sends the file to an Apple TV) so the .m4v extension seems to make more sense.
But iTunes can also play .mov and .mp4 files, so, adding those extensions to your iTunes Library folder helps keep things organized.