"What are you doing?"
It sounds like a scolding really, where it is kind of implied, that what you are doing is wrong :D Polite would be just "Mit csinálsz?" or if you are asking a specific person, "Te mit csinálsz?". Of course, with right intonation these all can be impolite, but that's the same as in English. Note that this is informal speech, so "Mit csinálsz?" could still be impolite if speaking in informal would be impolite.
Thank you for your quick reply. I asked because there was no “What am I doing,” in the English translation options for “Mit csinálok?” in Duolingo's Hungarian lesson. There were three answer options, two of which were clearly incorrect English. And the other one was “What are you doing”. When you selected this, it was the "correct answer". However, I think "Mit csinálok?" is "What am I doing," not "What are you doing," so I asked here. However, I don't think "Mit csinálok?" is "What are you doing," but "What am I doing," ..... so I asked here.