Standard Filing cabinets are not fireproof even though they are made of metal.
When you have a fire in a house or building and your files are in a standard metal filing cabinet the files and paper will be burnt.
The filing cabinet cannot protect the files and papers from fire like some other fire proof safes and cabinets can.
However they do make fireproof filing cabinets but they are not cheap.
However if you need to protect your very important documents from damage or even theft then the fireproof file cabinets can be worth the money.
A Fireproof File Cabinet costs around $1,000.00 and up depending on the size of the fireproof filing cabinet.
Here's a link to the maker of Fire Proof Filing Cabinets.
https://www.fireking.com/products/fire-rated-cabinets/fire-rated-file-cabinets