It's a vast field, and expertise of people is usually just a subset. I'm not even sure what the answer you you expected was, but I'd say: I'd use your public key to encrypt the file to you and then send it to you. Personally, I wouldn't know which commands to invoke to do this, but I know that's the theory.
So, should any developer know this? That is debatable. I've had very competent developers who had next to no clue about how DNS works. They could do their job just fine with that. Me? Personally, I'm not up to snuff with the finer points of SQL queries and all the joins that exists and when it makes sense to create an index, etc. Could I find out? Most likely, but I haven't had the need to recently.
The problem is, that you are mapping your knowlegde to "what people must know". I used to do that too, and I probably still do often enough. The DNS example above didn't come from nowhere: I had the case, and I was really thinking "how could such a competent person not know this", but then this person could probably enlighten me about dozens of things I don't know well enough.
It all comes down to what you define as "general knowgledge" for a developer should be and that is highly subjective.
TL;DR Hiring people is hard. Especially, technical people.