Appart from the fact that file filter seems to be broken, the file extension is corrupted under a certain circumstance:
1. Press "Save" push button. 2. Save using default name or a new name.gui (including extension) - file name is registered ok. 3. Press "save" push button again. 4. Enter any name without extension. 5. File name has the extension replaced by some weird characters.
It's confirmed. As soon as you add a non empty string to the "Default file extension" input, the primitive adds a wrong extension. But there is a workaround: Add a formated filter string to the component.