This function parses the HTML form data from a Rook environment (an HTTP POST request).
parse_multipart(env)
the HTTP request environment
A named list containing the values of the form data, and the files
uploaded are saved to temporary files (the temporary filenames are
returned). It may also be NULL
if there is anything unexpected in
the form data, or the form is empty.
This function was borrowed from https://github.com/jeffreyhorner/Rook/ with slight modifications.