In DNN 7.3.4 we are trying to file download and open but due to HttpContext.Current.Response.IsClientconnected showing false, Can anyone please help on this to resolve why HttpContext response connection is closed.

Webpage error details

Error Message: Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed. Common causes for this error are when the response is modified by calls to Response.Write(), response filters, HttpModules, or server trace is enabled.

Details: Error parsing near 'GIF89a￿,'.

Line: 3

Char: 74741

Code: 0

URI: http://localhost:8081/Telerik.Web.UI.WebResource.axd?_TSM_HiddenField_=ScriptManager_TSM&compress=1&_TSM_CombinedScripts_=%3b%3bAjaxControlToolkit%2c+Version%3d4.5.7.123%2c+Culture%3dneutral%2c+PublicKeyToken%3d28f01b0e84b6d53e%3aen-US%3ae669ce41-1aa1-4541-aae9-fa5dc37e70db%3aea597d4b%3ab25378d2