Hi Simon,
For more logging, you can increase the loglevel of REST* to debug or trace. JSON should not be uploaded to the Mendix app, but POSTed (you could upload and parse it, but that doesn't make much sense). Note that the contenttype header must be set to be able to upload and parse the json data. Currently that should be text/json, but in the next release this will be changed (backward compatible) to application/json.
Any REST client should suffice for testing, but personally I advice to start with curl and small pieces of data, because then your tests are easily shareable and you are sure you know exactly what is going on.
See also: http://www.mendix.com/blog/rest-part-ii-publishing-microflows-rest/