The Topfield is not a media player. If you want to watch something that is MPEG2, it is reasonably easy to convert the file to a Topfield format now, thanks to the good work of Deangelis etc. Unfortunately because their forums are down, I can't point you to the relevant threads though!!
Essentially, the process is to convert whatever you have into a MPEG-2 Transport Stream. Then you need to create the correct Topfield directory structure format which has header files for the file structure, and this is where I will have to stop, as I have a 5000 which supports a special "rec" header that can easily be added with projectX, where as the 7K series uses extra files for the headers instead, and some work with reverse engineering of the files was done by some smart 7K owners and they worked out enough to make a MPEG-2TS file playable on the 7K.