App::Document: fix file extension case sensitive problem

src/App/Document.cpp

@@ -2138,9 +2138,9 @@ static std::string checkFileName(const char *file) {
         const char *ext = strrchr(file,'.');
         if(!ext || !boost::iequals(ext+1,"fcstd")) {
             if(ext && ext[1] == 0)
-                fn += "fcstd";
+                fn += "FCStd";
             else
-                fn += ".fcstd";
+                fn += ".FCStd";
         }
     }
     return fn;