Can a hash of the original plain text file be used to brute force an encrypted copy?
It depends on the contents of your files. If the contents has high entropy and is unpredictable then you're probably safe.

However, if the contents is a password or a pin number, then brute forcing it should be trivial, it is for this reason that we use slow hashing. 

Another scenario might be if the contents had a value which was commonly known (ie, a pirated movie). If the hash of that file is common knowledge, then so is the contents of your encrypted file.

To err on the side of caution, it's probably better to ensure the integrity of your file with a HMAC or Authenticated Encryption. You might want to read this thread.