No, it didn't. The files are still mp3 files.
When Winamp is the default mp3 player it gives mp3 files the description of "MPEG Layer 3 File Format"
which is what mp3 stands for.
If you install an icon pack
, you can see that the files are still mp3
or if you set Windows not to hide extensions for known file types, you'll see that the file extension is still mp3
(see attached screenshot).
Alternatively, go to: Winamp -> Prefs (Ctrl+P) -> File Types
and deselect MP3 in the extension list.
Winamp will then no longer be the default mp3 player and whatever program was before should be instead.