I have a backoffice property editor that is a dropdown list that is meant to populate from a database. Here is the plugin controller, saved in the App_Code folder:

namespace My.Controllers
{
    public class Subscription
    {
        public int SubID { get; set; }
        public string Title { get; set; }
    }

    [Umbraco.Web.Mvc.PluginController("My")]
    public class SubscriptionPickerAPIController : UmbracoAuthorizedJsonController
    {

        public string GetAll()
        {
            List<Subscription> subs = new List<Subscription>();
            string conn = System.Configuration.ConfigurationManager.ConnectionStrings["WebSubscriptionsEntities"].ToString();


            conn = conn.Substring(conn.IndexOf("\"") + 1);
            conn = conn.Substring(0, conn.IndexOf("\""));

            using (SqlConnection connection = new SqlConnection(conn))
            {
                connection.Open();

                SqlCommand command = new SqlCommand("Select * from Subscriptions order by Title", connection);

                using (SqlDataReader reader = command.ExecuteReader())
                {
                    while(reader.Read())
                    {
                        Subscription thissub = new Subscription();
                        thissub.SubID = (int)reader[0];
                        thissub.Title = (string)reader[1];
                        subs.Add(thissub);
                    }
                }
            }

            string output = new JavaScriptSerializer().Serialize(subs);
            return output;      
        }
    }
}

I have the following AngularJS resource calling it, saved in the App_Plugins folder:

angular.module('umbraco.resources').factory('SubscriptionPickerResource',
    function ($q, $http) {            
        return {                
            getAll: function () {                
                return $http.get("backoffice/My/SubscriptionPickerApi/GetAll/");
            }
        };
    }
); 

My site runs over HTTPS. I have a redirect setup in IIS to direct all traffic to HTTPS. the web.config setting 'UmbracoUseSSL' is set to true.

I keep getting a 404 error. I have tried a bunch of different URL iterations, all come back 404. When I examine the request, I see the URL is 'https://mydomain.com/umbraco/backoffice/My/SubscriptionPickerApi/GetAll/'

What am I doing wrong?