Add dot only when the extension is not empty.

tools/assimp_cmd/Export.cpp

@@ -153,7 +153,9 @@ int Assimp_Export(const char *const *params, unsigned int num) {
     }
 
     // derive the final file name
-    out += "." + outext;
+    if (!outext.empty()) {
+        out += "." + outext;
+    }
 
     // and call the export routine
     if (!ExportModel(scene, import, out, e->id)) {