I have ran into similar issues. I ended up changing my HTML source files.
This also leads to the other issue, when you use LWIP you have think about how the system handles multiple requests for the same resource. That is if you are sending out a file can lwip have two open connections to the file? For example two computers accessing your server at the same time. This is one reason I implemented authentication system, as it restricts the number of connections, by restricting the number of users.