This is very confusing but this page is pretty clear :
"The MP4/M4A dual file extension thing is confusing. The thing to remember is
that M4A files will always contain only MPEG 4 Audio. MP4 files can be used for
MPEG 4 video files, combined video and audio files, or just plain MPEG 4 audio.
It is recommended that you use the .m4a file extension rather than .mp4 on your
.aac files are raw building blocks and will not be readable by iTunes/iPod
devices. This extension should not be used at all.
Think of M4A as a nice wrapper that contains an AAC file in it as well as other
details such as standard ID3 tag info etc..."
So, please close this ticket