Breaking encrypted file store with a file?
This is called the known-plaintext attack. Any cipher algorithm which is prone to this type of attack is considered very weak. Therefore, AFAIK, no present-day cipher (e.g. AES) has this weakness. Even DES needs 243 known plaintexts to be broken under linear cryptanalysis. (see this topic  on Wikipedia).

On the other hand, all hope is not lost. Weak ciphers or bad implementation of strong ciphers are still commonplace. One noticeable case is with the (old) implementations of ZIP. More info is available here:

http://www.elcomsoft.com/help/archpr/index.html?known_plaintext_attack_(zip).html