What is the ruling on swearing to break one's phone in a state of extreme anger, knowing that the cause of the anger was a mistaken assumption that one's sister had deleted personal files?

It is not permissible to break a phone, as it involves destroying property and transgressing against it. An expiation (kaffarah) for the oath is required due to the breach, based on the Prophet's (peace and blessings be upon him) saying: "Whoever takes an oath, then sees something else better than it, should do that which is better and expiate for his oath." And whoever swears an oath to do something forbidden must break that oath and expiate for it. Anger does not prevent the oath from being binding as long as the swearer is aware of what he is saying.