Why upstream_dns and upstream_dns_file cannot work as the same time?

[RMTT]

I checked the original commit message, but there was no information about that design.

I thought upstream_dns and upstream_dns_file can work togather, for example, making upstream_dns has higher priority.
It's really useful for some situations, for instance, i want to load a common domain rule file via upstream_dns_file and want to add some additional rules or override some rules in this file.