JSON doesn't have any built-in type that represents byte[]. On the source side, the converter is base64 encoding the data and storing it as a string, but then on the sink side there is no information about the schema which can be used to determine that we should reverse the base64 encoding and turn it back into a byte[].
This is just a fundamental limitation of JSON -- it loses schema information, so for schema types it does not natively support, there will be some mismatch between source and sink if the schema is not somehow carried over to the sink.
-Ewen