So, if you want to be able to upload mp4 files you have to tweak it a little bit.
When you setup the config arguments, add the allowed type and the max size.
$config['allowed_types'] = 'gif|jpg|png|mp4';
$config['max_size'] = '1000000000000000'; // whatever you need
Add the mime type mp4 to CodeIgniter
And add the following value to the $mime array.
'mp4' => 'video/mp4',
Now you should be able to upload a mp4 video through a form input 😉