toxster

I have a 2012 E350 4MATIC. It has an SD card slot for audio files. MP3 files work fine, but there is a problem when I copy AAC (.m4a) files from my computer to the card. For each file I put on the card, it generates another file which has the identical name except it is preceded by the characters ._ So if I have a folder of 10 songs, there appear to be 20 files in the folder when I navigate to it in the COMAND system. If there is a file called "01 Get Lucky" there will also be a file called "._01 Get Lucky." The extra files don't play, but they are there. The sound files play fine, but this is maddening when I am trying to navigate to a particular song. I cannot see these invisible files when the card is attached to my computer (even if I use a utility to unhide invisible files). Is this a vagary of the Mercedes-Benz audio system, or does somebody have a fix for this?