How to add data in session without lost of previous data of session.

Button bts = e.CommandSource as Button;
                if(e.CommandName.Equals("FileUpload"))
                {

                    FileUpload fu = bts.FindControl("fufileupload") as FileUpload;
                    objfile.filename = Path.GetFileName(fu.PostedFile.FileName);
                    objfile.extension = Path.GetExtension(objfile.filename);
                    objfile.contentType = fu.PostedFile.ContentType;
                    file = fu.PostedFile;
                    objfile.FileSize = file.ContentLength;
                    document = new byte[file.ContentLength];
                    file.InputStream.Read(document, 0, file.ContentLength);
                    DataTable dts = new DataTable();
                    dts.Columns.Add("Filename", typeof(System.String));
                    dts.Columns.Add("Extension", typeof(System.String));
                    dts.Columns.Add("ContentType", typeof(System.String));
                    dts.Columns.Add("FileSize", typeof(System.Int32));
                    dts.Columns.Add("document", typeof(byte[]));
                    DataRow drs=dts.NewRow();
                        //need to set value to MyRow column 
                        drs["Filename"] = objfile.filename;   // or set it to some other value 
                        drs["Extension"] = objfile.extension;
                        drs["ContentType"] = objfile.contentType;
                        drs["FileSize"] = objfile.FileSize;
                        drs["document"] = document;  
                  
                    //Listfile.Add(objfile.filename);
                    //Listfile.Add(objfile.extension);
                    //Listfile.Add(objfile.contentType);
                    //Listfile.Add(document);
                    //Listfile.Add(objfile.FileSize);
                        Session["GridRowFile"] = drs;
                    DataTable dt = new DataTable();
                    DataRow dr;
                    dt = (DataTable)Session["GridRowFile"];

                    dr = dt.NewRow();
                    dt.Rows.Add(dr);
                    Session["GridRowFile"] = dt;
                    //object list = (List<object>)Session["GridRowFile"];
                    //Session.Add("filegrid", list);


                }