Adds headers to response as described in wrap-file-info.

Wrap a handler such that responses with a file for a body will have
corresponding Content-Type, Content-Length, and Last Modified headers added if
they can be determined from the file.
If the request specifies a If-Modified-Since header that matches the last
modification date of the file, a 304 Not Modified response is returned.
If two arguments are given, the second is taken to be a map of file extensions
to content types that will supplement the default, built-in map.