Save as should save files using the same extension as the existing one

If I have e.g. an .opt or .bat file open in Creator and want to save a copy of it as another name, it will by default save it as .txt, appending that extension to the end of the file and forcing me to rename it again after saving it to remove the .txt extension. I can't think of any reason why this is the default behaviour. If I'm saving a copy of a file under another name, I almost always want to use the same extension.

A way of working around this is to choose "All files" in the file type combo box in the save dialog, but again, it seems very counter-intuitive to have to do this.