Tests:
- [Both overloads] uses the `download.Link` to guess the filenamem if it's not present
- [Both overloads] returns null if `download.Link` is null
- [Both overloads] returns nuyll if `download.RdName` is null
- [Both overloads] works for a file in a deeply nested subdirectory
- [overload with `downloadPath` argument only] creates the correct directory
