  ]])
    },
    -- second part: headers describe content to be a png image, 
    -- sent under the base64 transfer content encoding.
    -- notice that nothing happens until the message is actually sent. 
    -- small chunks are loaded into memory right before transmission and 
    -- translation happens on the fly.
    [2] = { 
      headers = {
        ["content-type"] = 'image/png; name="image.png"',
        ["content-disposition"] = 'attachment; filename="image.png"',
        ["content-description"] = 