can i sue a dentist for leaving a file in me withouth telling me

Legally, you may very well be able to sue, based on what you write; the question is, would it worthwhile to do so?
First, malpractice is the provision of medical (or dental) care which is careless or negligent, or which does not meet then generally accepted standards for such care. Leaving a foreign object (a file) in your mouth and/or not filing down a tooth woud appear to be just such a careless or negligent act, and so the dentist may be liable. 
However, you can only sue for an amount equivalent to the extent of your injury, with large awards generally requiring significant long lasting (months or longer) life impairment or disability; and for any out-of-pocket (not paid by by insurance or Medicare/caid) medical costs to correct the condition. At the same time, a malpractice suit can be a very expensive one to bring, because you really *should* have an attorney, and you MUST hire a medical expert (e.g. another dentist) to examine you, write a report, and testify (the court will not accept a non-expert's opinon, including yours, as to what is or is not accepted medical care, or as to the affect of the alleged malpractice); such medical experts can cost thousands of dollars. Since malpractice cases are so expensive, more minor, less life-impairing, or more easily remedied injuries may not result in sufficient awards or compensation as to justify the  cost of the suit.
Many malpractice attorneys provide a free initial consultation to evaluate a case--you can confirm this before making an appointment with the lawyer. A good idea would be to speak with a malpractice attorney; let an experienced lawyer advise you as to what your case might be worth, what it would cost to pursue it, and whether it is worthwhile.