In a!fileUploadField , I gave validation in sucha way that it allows only .xlsx format..,now my other requirement is that ,,,if the uploadedexcel doc is empty(if no data exists in cells),want to show no data available message how to acive this

To follow on from this you could make the User Experience synchronous by uploading the file, checking it's content and, if empty,  looping back to the same User Input Task interface (remembering to delete the invalid uploaded document when you finally have a valid document uploaded, just to keep your environment clean)