The length of the string exceeds the value set on the maxjsonlength property

Error during serialization or deserialization using the JSON JavaScriptSerializer. The length of the string exceeds the value set on the maxJsonLength property.
Parameter name: input

$.ajax({
           type: "POST",
           url: "/account/UploadServiceFilesAttachment",
           data: JSON.stringify({ OfferObj: $data }),
           contentType: "application/json; charset=utf-8",
           datatype: "json",
           success: function (result) {
               $service.AddServiceAttachment(result.Fullpath);
               dfd.resolve(result);
               return dfd.promise();
           },
           error: function (er) {

               var _er = er;
               return dfd.promise();
           }
       });

i have changed in web config 
<pre>   <jsonSerialization maxJsonLength="1073741824" />