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

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

--a----
Content-Type: text/plain

body
--a------
Content-Type: text/html

body2
--a----
---- EXPECTED STRUCTURE ----
{
  "header": {
    "content_type": {
      "c_type": "multipart",
      "c_subtype": "mixed",
      "attributes": {
        "boundary": "a--"
      }
    }
  },
  "html_body": [
    {
      "contents": "<html><body>body</body></html>"
    }
  ],
  "text_body": [
    {
      "header": {
        "content_type": {
          "c_type": "text",
          "c_subtype": "plain"
        }
      },
      "contents": "body"
    }
  ]
}