def multipart (self, fields):
        boundary = uuid4().hex
        blen = len(boundary)
        tlen = 0 # Length of the entire body
        w = lambda s, *d: s.write("".join(d))
        pre, mid, post = stringio(None), [], stringio(None)
        for key, value in fields.items():
            if hasattr(value, "read"): continue # First iteration: fields
            w(pre, "--", boundary, "\r\n")
            w(pre, "Content-Disposition: form-data; ")
            w(pre, 'name="', key, '"\r\n\r\n', value)
            tlen += 47 + blen + len(key) + len(value)
        for key, fd in fields.items():
            if not hasattr(fd, "read"): continue # Second iteration: streams
            name = getattr(fd, "name", key)
            ct = filetypes[name] or "application/octet-stream"
            wf = stringio(None)
            w(wf, "--", boundary, "\r\n")
            w(wf, "Content-Disposition: form-data; ")
            w(wf, 'name="', key, '"; filename="', name, '"\r\n')
            w(wf, "Content-Type: ", ct, "\r\n\r\n")
            mid.append(wf)
            mid.append(fd)
            tlen += 76 + blen + len(ct) + len(key) + len(name)
            tlen += fstat(fd.fileno()).st_size
        w(post, "--", boundary, "--\r\n\r\n")
        tlen += 8 + blen
        mid.append(post)
        c = iochain(None, pre, *mid)
        ct = "multipart/form-data; boundary=" + boundary
        return c, {"Content-Type": ct, "Content-Length": tlen}