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

--a
Content-Type: multipart/mixed; boundary="ab"
MIME-Version: 1.0
--ab
Content-Type: text/plain

--ab--
--a--


---- EXPECTED STRUCTURE ----
{
  "header": {
    "content_type": {
      "c_type": "multipart",
      "c_subtype": "mixed",
      "attributes": {
        "boundary": "a"
      }
    }
  },
  "attachments": [
    {
      "Text": {
        "contents": "--a--\n\n"
      }
    }
  ]
}