As far as I know the Humax is pretty fussy about the format it can stream, it is less fussy about the format it reads from a USB device. Also the DLNA protocol will only serve certain formats. I'm sorry I can't be definitive about what those formats are, but it has been discussed at length on the AVForums topic (refer to the Index in the pinned topics).
One problem is that something like AVI is actually a "container" for sub-streams for the sound and video, which can be many formats again.
The solution that others seem to have adopted is to use (IIRC) Twonky, which serves to a DLNA client and transcodes the video format on-the-fly.