If someone is able to break the encryption of a file its not very secure to begin with. However, just assuming that we have two files, one that is encrypted, and one that is encoded then encrypted, it would be trivial to break the encoding of a file within minutes versus days / weeks for certain types of encryption.
Encoding the file before encrypting it operates essentially the same as a substitution cipher, read here But the premise is that each individual character would be substituted or swapped for another value. History tells us that the first substitution cipher was written by Leon Battista Alberti in 1467, and was built on to create the Caesar Cipher. The link above gives a good deal more info on this sort of cipher.
So technically it might slow someone down since they would have to find out what the encoding is, and then substitute the values in, but if they made the effort to break the encryption you can be sure they'll be able to decode the file afterwards!