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Ajax Control Toolkit AsyncFileUploader Control and viewstate/session Issue
public partial class DesignUploader : System.Web.UI.UserControl
{
    private static readonly string AppDataPath = HttpContext.Current.Server.MapPath("~/App_Data/");

    public string FirstFilePath
    {
        get
        {
            return Server.UrlDecode( FirstFilePathHiddenField.Value);
        }
    }

    public string SecondFilePath
    {
        get
        {
            return Server.UrlDecode(SecondFilePathHiddenField.Value);
        }
    }

    protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);

        FirstFileUpload.UploadedComplete += FirstFileUpload_UploadedComplete;
        SecondileUpload.UploadedComplete += SecondileUpload_UploadedComplete;
    }

    void FirstFileUpload_UploadedComplete(object sender, AjaxControlToolkit.AsyncFileUploadEventArgs e)
    {
        var fullPath = Path.Combine(AppDataPath, Path.GetFileName(e.FileName));
        FirstFileUpload.SaveAs(fullPath);
        SaveFilePathToHiddenField(FirstFilePathHiddenField.ClientID, fullPath);
    }

    void SecondileUpload_UploadedComplete(object sender, AjaxControlToolkit.AsyncFileUploadEventArgs e)
    {
        var fullPath = Path.Combine(AppDataPath, Path.GetFileName(e.FileName));
        SecondileUpload.SaveAs(fullPath);
        SaveFilePathToHiddenField(SecondFilePathHiddenField.ClientID, fullPath);
    }

    private void SaveFilePathToHiddenField(string fieldId, string pathValue)
    {
        var script = string.Format("top.$get('{0}').value = '{1}';", fieldId, Server.UrlEncode(pathValue));
        ScriptManager.RegisterStartupScript(this, this.GetType(), "setPath", script, true);
    }
}