One-to-many file upload with additional checkbox

Link to test case:
Debugger code (debug.datatables.net):
Error messages shown:
Description of problem:
I have a one to many file upload setup following your example using a link table. My image model looks like the following

public class ImagesModel : EditorModel
    {
        public System.Guid ImageID { get; set; }
        public byte[] Content { get; set; }
        public string ContentType { get; set; }
        public string Extension { get; set; }
        public string FileName { get; set; }
        public int FileSize { get; set; }
        public string MimeType { get; set; }
        public DateTime CreatedDate { get; set; }
        public bool IsDefault { get; set; }
    }

Basically, I have "Products" that can have many images. This is all setup and working fine with all images currently being uploaded as "IsDefault" false. However, I need a way to mark the "IsDefault". I want to have a checkbox next to the image and then a validator in my controller to only allow 1 image to be checked but I'm struggling on how to accomplish this. Any suggestions? Here's my editor

using (var db = new Database(dbType, dbConnection))
            {
                var response = new Editor(db, "Product.Product", "ProductID")
                    //.Model<ProductModel>()
                    .Field(new Field("ProductID"))
                    .Field(new Field("Product.ProductID").Set(false))
                    .Field(new Field("Product.Identifier").Set(false))
                    .Field(new Field("Product.Title").Set(false))
                    .MJoin(new MJoin("Product.Images")
                        .Link("Product.Product.ProductID", "Product.ImageRef.ProductID")
                        .Link("Product.Images.ImageID", "Product.ImageRef.ImageID")
                        .Model<ImagesModel>()
                        .Field(new Field("ImageID")
                            .Upload(new Upload()
                                .Db("Product.Images", "ImageID", new Dictionary<string, object>
                                {
                                    //{"web_path", Path.DirectorySeparatorChar+Path.Combine("uploads", "__ID____EXTN__")},
                                    {"ImageID", Upload.DbType.ReadOnly},
                                    {"Content", Upload.DbType.ContentBinary},
                                    {"ContentType", Upload.DbType.ContentType},
                                    {"Extension", Upload.DbType.Extn},
                                    {"FileName", Upload.DbType.FileName},
                                    {"FileSize", Upload.DbType.FileSize},
                                    {"MimeType", Upload.DbType.MimeType},
                                    {"CreatedDate", Upload.DbType.ReadOnly},
                                    {"IsDefault", Upload.DbType.ReadOnly}
                                })
                                .Validator(Validation.FileSize(500000, "Max file size is 500K."))
                                .Validator(Validation.FileExtensions(new[] { "jpg", "png", "jpeg" }, "Please upload an image."))
                                )
                        )
                    )
                    //.Debug(true).TryCatch(false)
                    .Process(Request)
                    .Data();

                return Json(response);
            }

Note: I have IsDefault setup as a bit in my database and defaults to 0.

Thanks for any help you can provide.