What is the expiation for downloading files that are offensive to the Messenger, may Allah bless him and grant him peace, and presenting them to a non-Muslim friend?

You erred in your friendship with a disbeliever and did not act upon the Prophet's (peace be upon him) advice: "Do not accompany anyone but a believer, and let no one eat your food except a pious person." You also aided him in his disbelief and disobedience. As for your downloading the file without knowing its content, we hope you will be excused for that due to your ignorance.