Add a few external handlers for common file types.
- Can use "which" where available to figure out if preferred handlers are available. In some cases, may be able to use built-ins, e.g. mspaint can reliably be counted on to be there for Windows. Can Picture and Fax Viewer? I don't know.
- If no handler is registered, can default to downloads
- Leaning towards the "N" command opening a file, and having a "D" command to download. I.e. by default, you'd view a file by navigating to it; you could also choose to download it. If you couldn't view a file due to lack of association, downloading would be the default. This seems to match how most browsers today behave.