Import of grid models: Support mix of lower and upper case letters in file name

* #9529 libecl: Allow mix of upper and lower case in file names

ThirdParty/Ert/lib/ecl/ecl_util.cpp

@@ -355,7 +355,11 @@ static bool valid_base(const char * input_base, bool * upper_case) {
 
   if (upper_case)
     *upper_case = upper;
-  return !(lower && upper);
+
+  // Disable checking of mixed upper and lower case
+  // https://github.com/OPM/ResInsight/issues/9529
+  return true;
+
 }