You should not pay them for moral reasons (it proves to them that these attacks work) but from a selfish standpoint, paying them can get your files back. It is a toss up of whether or not they will give the files back. It has worked before, I have seen it first hand.
That said, technically if one file can be decrypted, they all should because the same encryption key was used to encrypt everything. (unless this is a new variant that uses multiple encryption keys. )