How To Show alert message when file already exists(When Browsing for a new file) in asp.net

/*To get user uploaded file name*/
UploadedFile filename = rauPIAMSpeRateLetter.UploadedFiles[0];
strGetFileName = filename.GetName();
 
                             /*Path to save user uploaded file(Store your path in hidden field)*/
                                string strFileName = hdfUploadPath.Value + strGetFileName;
 
                                /*Check for existence of file if not exists then I am saving else I am giving message to user*/
                                if (!File.Exists(strFileName))
                                {
                                    filename.SaveAs(strFileName, true);
                                }
                                else
                                {
                                    ScriptManager.RegisterStartupScript(this, this.GetType(), "Error", "alert('File already exist or rename file then upload');", true);
                                    return;
                                }