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

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

--ab
Content-Type: text/plain

body

---- EXPECTED STRUCTURE ----{
  "html_body": [],
  "text_body": [],
  "attachments": [
    2
  ],
  "parts": [
    {
      "headers_rfc": {
        "content_type": {
          "ContentType": {
            "c_type": "multipart",
            "c_subtype": "mixed",
            "attributes": {
              "boundary": "a"
            }
          }
        }
      },
      "headers_raw": [
        [
          {
            "Rfc": "content_type"
          },
          {
            "start": 13,
            "end": 44
          }
        ]
      ],
      "is_encoding_problem": false,
      "body": {
        "Multipart": [
          1
        ]
      },
      "offset_header": 0,
      "offset_body": 45,
      "offset_end": 131
    },
    {
      "headers_rfc": {
        "content_type": {
          "ContentType": {
            "c_type": "multipart",
            "c_subtype": "mixed",
            "attributes": {
              "boundary": "ab"
            }
          }
        }
      },
      "headers_raw": [
        [
          {
            "Rfc": "content_type"
          },
          {
            "start": 62,
            "end": 94
          }
        ]
      ],
      "is_encoding_problem": false,
      "body": {
        "Multipart": [
          2
        ]
      },
      "offset_header": 49,
      "offset_body": 95,
      "offset_end": 131
    },
    {
      "headers_rfc": {
        "content_type": {
          "ContentType": {
            "c_type": "text",
            "c_subtype": "plain",
            "attributes": null
          }
        }
      },
      "headers_raw": [
        [
          {
            "Rfc": "content_type"
          },
          {
            "start": 113,
            "end": 125
          }
        ]
      ],
      "is_encoding_problem": true,
      "body": {
        "Text": "body\n"
      },
      "offset_header": 100,
      "offset_body": 126,
      "offset_end": 127
    }
  ]
}