I have a Lumia 930 and I use Nokia Camera App to take my video. How can I convert the mp4.tnl files?
The mp4.tnl file extension in a file in Windows Phones represents packages in which the raw image files and data are stored by the Lumia Camera, for the purpose of animating "Living Images". If you go under Files, you will see that for an image taken with the Lumia Camera, there exists 2 or 3 other files, with the same name as the actual JPEG image, but with different extensions, .mp4.tnl being one of those:
Importantly, the presence of these two extra files don't appear to get in the way of other usual photo operations on the phone.
So in essence, you don't need to convert them, per se. Just a file manager on your computer will do the trick to open the package to see whats inside.