This problem has been reported a number of times, but it doesn't look like anybody cares about. It looks like this happens only to very few users, because there would be more complaints about it if it was more widespread.
Here is my description of the problem: /t/10398/1
And here is a workaround solution: Not possible to change filename w/ extension via file view.
Another workaround solution is by using a bat file with a REN command for a tool in Mp3tag.