If you're talking about resources, it makes more sense to make that file it's own resource.
map.resources :foos do |foo| foo.resource :download # assuming there's one 'dynamic' file per Foo record end
GET /foos/1/download # downloads/index action.
Part of the rails restful routing style was to stick to the core 7 actions (index, show, new, edit, update, create, destroy) by bringing out the "inner resource". This makes more sense if folks can also update or destroy the extra resource. Then you wouldn't need several actions like show_download, create_download, etc.