Decryption or attacking DES encrypted file
Brute force is feasible as codesinchaos points out. That said there is more to it than that.

First you'll need some way to tell whether or not decryption was successful. For example, you may look for english language text upon decryption. Automating this will be essential as you don't want to manually look at every decryption to decide whether or not you were successful.

Second, you'll have to try different modes. If it was encrypted with ECB, you should be able to recognize that by looking at the ciphertext (see if there are lots of repeated blocks of ciphertext; a block in des is 8 bytes). Otherwise, you'll have to try all the common modes. Still, should be able to be accomplished via brute force.