Stop replacing digits after dots in set_filename_extension()

Utils/FileOutput.cc

@@ -7,6 +7,7 @@
 #include <iomanip>
 #include <fstream>
 #include <limits>
+#include <cctype>
 
 namespace Base
 {
@@ -29,11 +30,17 @@ std::string make_filename(
 std::string set_filename_extension(const char* _flnm, const char* _ext)
 {
   std::string flnm(_flnm);
-  const size_t dot_pos = flnm.find_last_of('.');
-  if (dot_pos != std::string::npos)
-    flnm.replace(flnm.begin() + dot_pos + 1, flnm.end(), _ext);
+  const auto dot_pos = flnm.find_last_of('.');
+
+  if (dot_pos == std::string::npos)
+    flnm += std::string(".") + _ext; // no dot, add the dot and the extension
+  else if (dot_pos + 1 == flnm.size())
+    flnm += _ext;                      // last char is dot, add the extension
+  else if (isdigit(flnm[dot_pos + 1])) // char after dot is a digit?
+    flnm += std::string(".") + _ext;   // add the dot and the extension
   else
-    flnm += std::string(".") + _ext;
+    flnm.replace(flnm.begin() + dot_pos + 1, flnm.end(), _ext);
+
   return flnm;
 }