Hi ashutosh
I don't think I can help - but I'm curious the datatype defined in the response is application/ms-excel?
I can see .html you mention in your RAW - but what is present in your outline tab - is it the .html? if it's .html can't you use xpath to do your assertions?
My only other option and I doubt this will work is you can try using the Accept header on the request that generates the response.
I use the 'Accept' header (value = application/json or application/xml) to switch the type of the response from .json to .xml or .xml to .json
I don't think you can change .xlsx to something else (all the markup would get in the way I'm guessing) - but if the response includes .html - I don't see why you can't switch it to .xml or .json.
Obviously this is dependent on your application server.
Doubt I've helped - there's probably other reasons why it won't work that I don't know - but I'd give it a try anyway, just in case.
Cheers,
richie