You can't just change the naming convention. Won't work. The video must be re-compiled using specific video codecs applicable to the format.
There are apps that can convert these, but mostly PC based.
If you were recording your video at the highest resolution, you could try dropping to the next lower setting (if you have the option), this will increase the frame rate and smooth it out.
Or, you can download a different video player. MX Player seems to work well for most folks. You have a choice between hardware and Software decoding. Give it a shot.