[galaxy-dev] Conditional tool options based on input file format?

Dear all, I've looked over the wiki and as far as I can see, 'when' tags used inside 'conditional' tags only work on another input variable's values. I would like to be able to do is a conditional based on the file format of an input file. For instance, a tool might take XML or tabular files, and need to show additional parameters for one input type but not the other. Does this enhancement idea seem useful? Peter