copy all the extensionless files to a separate folder(i.e. c:\badmp3).
Open a command prompt window.
go to that folder:
c:
cd\badmp3
then type:
ren *. *.mp3

That should rename all files with no extension to the same name with the mp3 extension. I don't think you will lose the long file names but just in case make sure you copy the extensionless files instead of moving them. If it all check out, delete the original extensionless files and copy the newly extensioned files to the folder where they came from.

Hope this helps.

Rene