I want MP3 but AME converts WAV to MP4

I have a set of WAV files loaded that I want to convert to MP3. I select verious choices of presets under the Audio heading that say MP3 but everyone lists the resulting file extension in the Queue as MP4. And sure enough, they convert to MP4, but every time you play them you see a (black) video window. You do hear them, but I don't want a window to open every time. Why can't it convert to MP3 if I ask for MP3?