How to use Json.NET for model binding in mvc 5

I have the same issue as this guy posting single item in json array to .net object.

To summarize, I am sending JSON data to an action and getting null when there is only one item passed. But it works when there is more than one item.

I know this has to do with how the JSON string is constructed (list of object vs. single object) but I have no choice; I am using a limited jquery json library (that's another story)

JSON string example with a list of fields

{  
   "formtemplate":{  
      "fields":{  
         "field":[  
            {  
               "_class":"form-control text-input",
               "_label":"Text Field",
               "_name":"text-1467980984393",
               "_type":"text",
               "_subtype":"text"
            },
            {  
               "_class":"form-control text-input",
               "_label":"Text Field",
               "_name":"text-1467980999418",
               "_type":"text",
               "_subtype":"text"
            }
         ]
      }
   }
}

JSON string example with only one field item

{  
   "formtemplate":{  
      "fields":{  
         "field":{  
            "_class":"form-control text-input",
            "_label":"Text Field",
            "_name":"text-1467980984393",
            "_type":"text",
            "_subtype":"text"
         }
      }
   }
}

Model

public class Fields
{
    public List<Field> field { get; set; }
}

public class Formtemplate
{
    public Fields fields { get; set; }
}

public class CustomFields
{
    public Formtemplate formtemplate { get; set; }
}

public class Field
{
    public string _label { get; set; }
    public string _name { get; set; }
    public string _type { get; set; }
    public List<Option> option { get; set; }
}

Action

[HttpPost]
public JsonResult saveCustomfields(CustomFields customfields)
{

}

I followed the advice on the link provided but it didn't seem to work, here is what I did

public class Fields
{
    private List<Field> field;

    public object Numbers
    {
        get { return field; }
        set
        {
            if (value.GetType() == typeof(Field))
            {
                field = new List<Field> { (Field)value };
            }
            else if (value.GetType() == typeof(List<Field>))
            {
                field = (List<Field>)value;
            }
        }
    }
}

In the action I get both of "Numbers" and "field" as null when there is one item or more. The solution is logical but it didn't work for me.

Update

I researched a bit more and implemented a custom converter from this link How to handle both a single item and an array for the same property using JSON.net. But when I debug; it seems like SingleOrArrayConverter is not even called

public class Fields
{
    [JsonProperty("field")]
    [JsonConverter(typeof(SingleOrArrayConverter<Field>))]
    public List<Field> field { get; set; }
}

public class Formtemplate
{
    [JsonProperty("fields")]
    public Fields fields { get; set; }
}

public class CustomFields
{
    [JsonProperty("formtemplate")]
    public Formtemplate formtemplate { get; set; }
}

public class SingleOrArrayConverter<T> : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(List<T>));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JToken token = JToken.Load(reader);
        if (token.Type == JTokenType.Array)
        {
            return token.ToObject<List<T>>();
        }
        return new List<T> { token.ToObject<T>() };
    }

    public override bool CanWrite
    {
        get { return false; }
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

But still no success, does it have to do with my action?