log files cannot be converted to .txt

[ibramedia]

I found an error in the configuration in the config.php file on the log_file_extension line it was set to .txt but the file was still created .php

[pocketarc]

This config was removed from CodeIgniter in 3.2 (upstream, not this repo); there is no more support for .txt logs.

This might be something worth reverting here, for compatibility.