Is it permissible to create a file hosting website for a fee, knowing that I do not know the content of the uploaded files, and it is most probable that the most downloaded files may not be compatible with Islamic ethics?

Overwhelming probability is treated as certainty. So, if you have an overwhelming probability that the files contain forbidden matters, then it is not permissible to host them. This is because doing so would be an assistance in sin, and Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty."