Sample sentence: If anyone has the files, could (they or you) please upload them to a file-hosting website?
Context: I am writing an email to a group of people requesting that if anyone has the files I mentioned, to please upload those files to a file-hosting website. "Anyone" could be one person or more.
Surely, my first choice out of your dilemma would be to change the sentence slightly and write "if anyone has the files, please upload them to a file-hosting website".
Otherwise, although I feel that they would be the right pronoun to use to refer to anyone, because you don't know how many people could have the file and also because you don't know whether these people are men or women, still I would write you in the sentence you mention, because the context is rather friendly and they might be interpreted as overly rigid and formal.
Nothing wrong with they. As the OED tells us, they is ‘often used in reference to a singular noun made universal by every, any, no, etc.’ It was good enough for, among others, Henry Fielding, Lord Chesterfield, Walter Bagehot and John Ruskin – and indeed is for most native speakers of English today.
