ignore nested Content-Type inside multipart/form-data

multipart/form-data uses key-value format in the http request body
example: key1=value1&key2=value2&key3=value3

you can use irules to check that pattern in request body but in this case the ideal solution is filtering by webserver.
they can use "try catch" mechanism or check of "undefined" in the server application code.
basically, make the server application code to be foolproof.