Shall a (rest) API receive JSON or "standard" POST params?

This topic came up today when we discussed the #crewmeister API, and made me think. Until I remembered that we once in a project used to use the pure POST params, which are basically key-value pairs. And the question also reminded me of my PHP times, there the server-side knew how to handle a key which looks like user[0] and builds and array. But when I ran into that problem back then we used to use python and there we didn't have that.

Final answer: Use JSON where possible! There might be some overhead (if you don't use nodejs) on the backend, because you will need a JSON decoder. But therefore you have a well-defined format for receiving the data. Transfer binary data, like image uploads in separate requests encoded accordingly and you will be fine, I think.