         boundary = new_boundary()
 
   """
+  @spec new_boundary() :: boundary_t
   def new_boundary, do: "---------------------------" <> unique(16)
 
   @doc """

  defp mp_mixed_header(name, boundary) do
    headers =
      [{"Content-Disposition", "form-data", [{"name", "\"" <> name <> "\""}]},
       {"Content-Type", "multipart/mixed", [{"boundary", boundary}]}
      ]
    {mp_header(headers, boundary), 0}
  end


          8. `{name, bin_data, disposition, extra_headers}`

  """
  @spec encode_form(boundary :: boundary_t, parts :: [part_t]) :: {boundary_t, integer}
  def encode_form(boundary, parts) when is_list(parts) do
    encode_form(parts, boundary, "", 0)
  end

