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

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

--a
Content-Type: text/plain

body
--ab
Content-Type: text/html

body2

---- EXPECTED STRUCTURE ----
{
  "headers": {
    "content_type": {
      "ContentType": {
        "c_type": "multipart",
        "c_subtype": "mixed",
        "attributes": {
          "boundary": "ab"
        }
      }
    }
  },
  "attachments": [
    {
      "Text": {
        "headers": {
          "content_type": {
            "ContentType": {
              "c_type": "text",
              "c_subtype": "plain"
            }
          }
        },
        "contents": "body\n--ab\nContent-Type: text/html\n\nbody2\n"
      }
    }
  ]
}