When the anticache option is passed to mitmproxy, it removes headers (if-none-match and if-modified-since) that might elicit a 304-not-modified response from the server. This is useful when you want to make sure you capture an HTTP exchange in its totality. It's also often used during client replay, when you want to make sure the server responds with complete data.

command-line --anticache
mitmproxy shortcut o then a