Challenge: fastest program wins

Write a program in your language that
1.opens a file as arbitrary precision integer.
2.divides the number by 3.
3.stores the binary result in another file.

>>86
Obviously, it means take the actual file data to be a number.
`Obvious' is probably not the word you're looking for here considering every single program in this thread does it the other way.

Given >>1's definition, how would you interpret a binary file as an arbitrary-precision integer?