This function parses the HTML form data from a Rook environment (an HTTP POST request).
parse_multipart(env)
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.
the HTTP request environment
This function was borrowed from https://github.com/jeffreyhorner/Rook/ with slight modifications.