one step in command prompt
the following will work just fine to convert extension (replace with blah) into mp3 extensions.
>rename *.blah *.mp3
if you want to save it as a bat file,
1.open notepad
2.type the following rename *.blah *.mp3
3.save file as "convert.bat" . make sure the quotes are there.else it will be saved as convert.bat.txt
4.double click on convert.bat or go to command prompt and open the file by simply typing ~dir>convert