Uh yes, you are right! The problem is that the “Write Binary File” expects binary data. The JSON data is, however, JSON 
There should really be a node that allows moving data around between both of them.
For now, it can be done with a Function-Node. At least till a proper one is in place. Pasted an example of how you can do that to the end of the message. The first node you can remove, it is only to create the example data. The second node named “Make Binary” is the important one with the custom code which makes the data binary and writes it to the correct location.