URGENT!!Exporting mp3 files?

Alright, so I’ve recently tried to create an off-vocal version of a song, and I have tried to export it as an mp3 file, but everytime I try, the exported file is not an mp3 file, but just a file.
Is there any way to solve this problem?

By reading the FAQ’s in the Manual :wink: ?
http://manual.audacityteam.org/o/man/faq_opening_and_saving_files.html#extensions .