Leezer, thanks for the trick.
Nathan, thanks for the info. Your way seems simpler.
Yesterday when I tried to change an iso to Video rather than Data, the change didn't stick. But it must have been a glitch, because this morning it is sticking.
I would of course prefer to have it done automatically, which is what I thought I was doing by adding the file types to the "video" section of the xml.
I did perhaps find a bug: I thought that tag changes to stacked items affected all items in the stack, but that doesn't happen with a change in Media Type. Most of my iso files have corresponding mds files. I didn't want duplicates in my Movie View. I could have created an expression saying: when both files exist, show only the mds, otherwise show the iso, but I couldn't for the life of me come up with a successful expression to do that, so I stacked them instead. But mofidying the Media Type doesn't apply to the item 'under' the stack.