The REST server (or rather web application) does not properly encode binary values in the JSON results, and instead is embedding the toString() version of the byte[]. The binary values should instead be encoded in Base64, and the REST client should be able to then read (or upload) binary values using this approach.

(If the binary value is already a Base64-encoded representation, then the JSON content will just be a Base64-encoded representation of the value that happens to be Base64-encoded. Shouldn't be a problem.)