Not able to import a .m4a audio file into a lesson

I am trying to create a lesson with a .m4a audio file. I try to import the file and everything looks ok. I can even play the file and hear audio ok. But when I try to generate the lesson, I get a message that the file type is not supported. How can I get my audio file imported?

At the moment, only mp3 audio files are supported, mp4 file can’t be uploaded.