The video/x-ms-asf mimetype is valid for .asf, .wmv and .wma files
but as we can't map more than one extension to the same mimetype I've
added a little hack allowing the .wmv and .wma extensions to also
include the video/x-ms-asf mimtype in the list of valid mimetypes.

Change-Id: I706d86c9bc0c5d0c0eb6ab0a09b1f16208557b00
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>