> ...there are ways to produce request bodies that are valid JSON even if the browser forces you into a different format...

The browser basically never forces you into a particular format. You don't even need to do the trick with the form stuff that the sibling was talking about. Consider the following JavaScript:

    var xhr = new XMLHttpRequest();
    var url = "http://localhost:12345/endpoint";
    xhr.open("POST", url, true);
    xhr.setRequestHeader('Content-Type', 'multipart/form-data');
    xhr.send('{"hello":"world"}');

No trickery required, it just does it.

[Edited to illustrate my point better.]