If filing the teeth is done for the purpose of increasing and enhancing beauty, then it is haraam; if it is done to treat a medical problem or to remove a fault, such as a tooth that is longer than the other teeth, then it is permissible. 

The basic principle concerning that is the report narrated by al-Bukhaari (5943) and Muslim (2125) from Ibn Mas‘ood (may Allah be pleased with him) who said: May Allaah curse the one who does tattoos and the one who has a tattoo done, the one who plucks eyebrows and the one who has her eyebrows plucked, and those who file teeth for the purpose of beautification, changing the creation of Allaah. 

An-Nawawi (may Allah have mercy on him) said in Sharh Muslim: 

What is meant by “those who file teeth” is those who file the four front teeth; that is done for old women and those who are approaching old age, to make them appear younger and to make the teeth look more beautiful, because this small gap between the teeth is for younger girls; when a woman gets old her teeth become bigger, so they file the teeth to make them appear more beautiful and to make people think that they are young. This action is haraam both for the one who does it and the one who has it done to her, because of these hadeeths and because it is changing the creation of Allah, may He be exalted; and that is also because it is a kind of deceit. 

With regard to the phrase “those who file teeth for the purpose of beautification”, what is meant is that they do that for the sake of looking beautiful. This indicates that what is haraam is that which is done for the purpose of beautification. But if a woman needs that to treat a problem or fault in the tooth and so on, then there is nothing wrong with that. And Allah knows best. End quote. 

See also the answer to question no. 1006 

And Allah knows best.