I hope you found a solution for this question before now but if you didn't here is how I found where the files are stored. I imagine you have Admin access to your account, or that the account you are using has "sudo" rights. If your account has "sudo" rights (which you can test by running the command below) this will find the things for you. The code below first changes into the root directory on your system and then runs the query to look for the mp4 files (for avi just replace mp4 with avi):
sudo find . -iname '*.mp4'
This should give you a full path for every mp4 file on your system. It is going to take a reasonable amount of time to go through the whole system. On my system, files were stored in one of the subdirectories of /private/var, so what you can do to make that 'find' command go a bit faster cd into '/private/var' instead of '/' in that first command above.
If you don't have sudo access, you can still run the command 2 but remove 'sudo'. It will go through directories again, but you will only have results for files that you have access to. You can also log in as Administrator user and run the commands above (without 'sudo', just 'find' portion).
I hope this helps you and others who have the same question.