Submenu name: the model we're pushing (according to @minrk) is that the file on disk is always up to date, and user interaction is only for checkpoints. So I think calling it 'save and download as' would be confusing (e.g. is that '(save and download) as'?)

I'm not convinced about building it dynamically: I think there will be exporters exposed that you don't necessarily want in the menu, in which case it's an extra level of complexity to configure which ones are exposed. We may later want to dynamically list some options depending on whether the tools are there to support it (e.g. list PDF if LaTeX is installed), but I think that's best done in a separate PR.