The DNS-323 had (has?) a similar incompatibility issue with MKV files. The following thread describes the DNS-323 issue and other compatible video formats: http://forums.dlink.com/index.php?topic=14731.0.
Assuming this issue in the DNS-320 is similar to the DNS-323, then you'll have to convert your videos to another container/codec. See the referenced thread.
Please let us know whether this information helps you.