AngularJS $http / .NET Web API - Either Getting 400 (Bad Request) or NULL POST Action Method Parameter

Here’s a brief (as I can make it) description of my problem, along with all relevant code. I'm hoping the wording for this post will be a bit clearer than my previous request for assistance.

I have a .NET Web API, and an AngularJS front end. I have a very simple POST method which accepts a parameter of the ‘Envelope’ type, shown here:

public class Envelope {
    public int listingId { get; set; }
    public string Description { get; set; }

    public override string ToString() {
        return listingId.ToString() + "; " + Description;
    }
}

The actual POST method on the API appears here:

[EnableCors(origins: "http://simpleapiearl.azurewebsites.net", headers: "*", methods: "*")]
public class EnvelopesController : ApiController {
        // POST: api/Envelopes
        public string Post(Envelope env) {
            return "rval: " + env.ToString() + " (and my addition to env)";
        }
    }

My front-end AngularJS $http POST looks like this:

        $scope.testPOST = function () {
            var env = {
                listingId:1234,
                Description:"some desc"
            };

            $http({
                method: 'POST',
                url: 'http://simpleApiEarl.azurewebsites.net/api/envelopes',
                data: JSON.stringify(env),
                headers: {
                    'Content-Type': 'application/json'
                }
            }).
            success(function (data, status, headers, config) {
                $scope.postStatus = 'success: ' + data;
            }).
            error(function (data, status, headers, config) {
                $scope.postStatus = 'error: ' + status;
            });
        }

Here are my issues (numbered for easier reference):

1. Using all the code as shown above, I get a “400 (Bad Request)” when I call “testPOST()” from my page. This sounds like a .NET Web API routing issue, but I can’t figure out what it is.
2. I can avoid the 400 (and in fact get a 200) if I change the ‘Content-Type’ header to ‘application/x-www-form-urlencoded’. HOWEVER, that results in the API seeing the ‘env’ parameter as NULL.
3. I tend to adorn my action method parameter with a ‘[FromBody]’ attribute, but doing so does not fix the problem of ‘env’ being NULL in my API action method.

I have created a simple plunk with my very simple HTML page used to call the API. It can be found here:

http://plnkr.co/edit/hY2OUeg9CRQ1QOz8MGU8?p=info

Thanks very much for any assistance you can provide.