Content-Type: multipart/mixed; boundary="1"

--1
Content-Type: text/plain

hello, world
--1

---- EXPECTED STRUCTURE ----
{
  "headers": {
    "content_type": {
      "ContentType": {
        "c_type": "multipart",
        "c_subtype": "mixed",
        "attributes": {
          "boundary": "1"
        }
      }
    }
  },
  "html_body": [
    {
      "contents": "<html><body>hello, world</body></html>"
    }
  ],
  "text_body": [
    {
      "headers": {
        "content_type": {
          "ContentType": {
            "c_type": "text",
            "c_subtype": "plain"
          }
        }
      },
      "contents": "hello, world"
    }
  ]
}