This is lower case output (file extension is same).
Find:
return isset($regs[1]) ? $regs[1] : $basename;
replace:
return isset($regs[1]) ? strtolower(trim($regs[1])) : strtolower(trim($basename));
After - find:
return strtolower($regs[2]);
replace:
return strtolower(trim($regs[2]));