I had problems with the MP4 association (I think an audio player somehow
took it over from WMP). Anyway, at some stage I had no entry for it in
File Types. I tried adding one, using New... but it wouldn't 'stick'. So
then I used Open With, chose the list option, selected WMP, and check
marked 'Always use this program'. That works in the sense that d-clicking
an MP4 now opens it in WMP. But why is there still no entry for MP4 in
File Types please?