Error while uploading, HTTP Error code is: 500 while uploading file in RadAsynUpload Control

It's our Enterprise app, so cant update it here. 

Our Master Page has a Script Manager, so I didnt use another scriptmanager for Radcontrols in my aspx page.

Here is my scriptmanager piece in Master page.

  <asp:ScriptManager ID="ScriptManagerMain" runat="server" EnablePageMethods="true"
            EnablePartialRendering="true" EnableHistory="true" EnableSecureHistoryState="False" AsyncPostBackTimeout="300">

Here is the control I have in my aspx page.

  <telerik:RadAsyncUpload runat="server" ID="RadAsyncUpload1" HttpHandlerUrl="~/FileUpload/FileUpload.ashx"
            OnClientFileUploaded="fileUploaded" OnClientFileUploadRemoving="fileUploadRemoving"
            MultipleFileSelection="Disabled" AllowedFileExtensions="jpeg,jpg,gif,png,bmp" Width="500px" />
        <div class="imageContainer"></div>

Here is the Javascript file.

(function () {
  var $;
  var demo = window.demo = window.demo || {};

  demo.initialize = function () {
    $ = $telerik.$;
  };


  window.fileUploaded = function (sender, args) {
    var id = args.get_fileInfo().ImageID;

    $(".imageContainer")
        .html("")
        .append($("<img />")
        .attr("src", getImageUrl(id)));

    $(".info").html(String.format("<strong>{0}</strong> successfully inserted.<p>Record ID - {1}</p>", args.get_fileName(), id));

  };

  window.getImageUrl = function (id) {
    var url = window.location.href;
    var handler = "FileQuery.ashx?imageID=" + id;
    var index = url.lastIndexOf("/");
    var completeUrl = url.substring(0, index + 1) + handler;
    return completeUrl;
  };

  window.fileUploadRemoving = function (sender, args) {
    var index = args.get_rowIndex();
    $(".imageContainer img:eq(" + index + ")").remove();
  };
})();