When reading a multipart/form-data POST body, assume the response

When reading a multipart/form-data POST body, assume the response
encoding as external format.  Firefox does not send explicit encoding
information with m/f-d bodies, and instead assumes the encoding of
the page containing the <form> element.  This can fail if the page
is POSTed to a different server, but it works better than the static
+latin-1+ default that was used before.