Return the per-file header as a string.

def FileHeader(self):
        """Return the per-file header as a string."""
        dt = self.date_time
        dosdate = (dt[0] - 1980) << 9 | dt[1] << 5 | dt[2]
        dostime = dt[3] << 11 | dt[4] << 5 | (dt[5] // 2)
        if self.flag_bits & 0x08:
            # Set these to zero because we write them after the file data
            CRC = compress_size = file_size = 0
        else:
            CRC = self.CRC
            compress_size = self.compress_size
            file_size = self.file_size

        extra = self.extra

        if file_size > ZIP64_LIMIT or compress_size > ZIP64_LIMIT:
            # File is larger than what fits into a 4 byte integer,
            # fall back to the ZIP64 extension
            fmt = '<HHQQ'
            extra = extra + struct.pack(fmt,
                    1, struct.calcsize(fmt)-4, file_size, compress_size)
            file_size = 0xffffffff
            compress_size = 0xffffffff
            self.extract_version = max(45, self.extract_version)
            self.create_version = max(45, self.extract_version)

        filename, flag_bits = self._encodeFilenameFlags()
        header = struct.pack(structFileHeader, stringFileHeader,
                 self.extract_version, self.reserved, flag_bits,
                 self.compress_type, dostime, dosdate, CRC,
                 compress_size, file_size,
                 len(filename), len(extra))
        return header + filename + extra