As Yusuke asked, is the "raw data" referring to specific value in the JSON payload that is base64 encoded? Or do you just want to display the JSON payload in a different more readable format?
In the case you're talking about the first option then you can use a JSON Path Filter to extract the actual "raw data" value. You can then add another step (Execute Script) to decode the raw data into readable string:
import org.apache.commons.codec.binary.Base64;
String raw_data = testExec.getStateValue("rawDataProperty");
byte[] encodedBytes = raw_data.getBytes();
byte[] decodedBytes = Base64.decodeBase64(encodedBytes);
String decodedReadableValue = new String(decodedBytes);
testExec.setStateValue("decodedReadableValue", decodedReadableValue);
return decodedReadableValue;
If this is regarding the second option then I'm not aware of an out of the box feature that will transform the JSON payload to another format. A custom parser may have to be developed or perhaps make use of existing open source projects.