Why Cant I See Mkv Files On The Whs2011?

Hi,

I have two Samsung DLNA 'smart' tvs &  a western digital live device for a third tv.  I notice if i put an AVI file on teh WHS it will play on all devices.  If I put a MKV file it wont work.  I downloaded somethign called playstation dlna (or something like that) for a windows 7 laptop & all three devices can see the MKV file no problem.

How do i fix the WHS so it sees the mkvs & can play them?

Thanks,
Rich