Hi zafer,
The monitoring daemon evaluates the receive string against the response content. So it shouldn't matter what type of file you configure for the request as long as the server responds with the expected content.
If this isn't working, you could either enable bigd debug (b db bigd.debug enable) and check the /var/log/bigdlog file, or you could test from the command line using curl. If you make a curl request from LTM to a pool member, do you get back a response containing "hello"? You can use something like 'curl -v http://1.1.1.1/id/123445' to test where 1.1.1.1 is the pool member IP address.
Aaron