extjs - ext js file download or JSON process conditionally depending upon MEDIA type -

i using jersey ext js in front end, submitting form using ext.fly

ext.fly(form).set({     id : 'downloadform',     target: hiddeniframe,     method: 'post',     action: downldobj.url });   form.submit(); 

it works fine file download popup in browser. requirement when try send conditionally json response

return response.status(status._statuserror).entity("{success:false,result:'failed'}").type("application/json").build(); 

ext js cannot identify json. there anyway in ext- js can process json response or prompt download conditionally.