If you are not sure it should be by default set to file. But either way please check in include/config.php search for this line
if(!defined('SESSION_TYPE')) define('SESSION_TYPE'
unless you are overwriting this somewhere in the code, so the best way will be to go to
http://<you_epesi_url>/admin
and in 'run php command' paste
print SESSION_TYPE;
this way we will know for sure
