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

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

--ab
Content-Type: text/plain

body

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