Webgui doesn't know of ALL file types, so you can add your own:

1. find an icon that is 16x16 pixels. It can be of any format.
2. Put the icon in the /data/WebGUI/www/extras/fileIcons folder
3. Modify the /data/WebGUI/www/extras/fileIcons.js file and add an entry for each file type:
   

   "m4a"       : extras + "fileIcons/m4a.gif",

This change applies to all sites, and does not require a restart of any service.