Problem downloading files under https

 Hi,
I have a page that presents an image with onClick code to download a pdf/Excel file. The OnClick code I'd like to use is something like
Page.ClientScript.GetPostBackEventReference(this, mystring)
which I would then receive in a RaisePostBackEvent method to serve the file, like this:

                 HttpContext.Current.Response.Clear();
                HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.HtmlEncode(file.Name) + ";");
                HttpContext.Current.Response.AddHeader("Content-Length", file.Length.ToString());
               ...