Steps to reproduce the issue

A quick pseudo code:
JS:

var data = new FormData();
data.append('foo', 'bar');

Joomla.request({
 url: 'bla/bla/request/url',
 data: data
});

PHP side:

var_dump($_POST);

Using FormData: https://developer.mozilla.org/en-US/docs/Web/API/FormData/Using_FormData_Objects

Expected result

The dump should contain array with foo => bar

Actual result

The dump contain random string;

A reason

The reason of this bug is that Joomla.request force 'Content-Type', 'application/x-www-form-urlencoded' for all POST data:

However the Content-Type should be multipart/form-data; boundary={boundary string}. By default browser will set it correctly unles other specified.

So we should not force application/x-www-form-urlencoded' for all POST data.