Yes, as I said above the KMS Encryption / Decryption used will be done on S3 side.

Depending of the component you are using, you're either going to decrypt the file or encrypt it.

tS3Get will Decrypt using KMS

tS3Put will Encrypt 

Give it a try and you'll see

Don't forget to mark the post as solved if everything is ok for you 

- Quentin