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_rfc": {
    "content_type": {
      "ContentType": {
        "c_type": "multipart",
        "c_subtype": "mixed",
        "attributes": {
          "boundary": "ab"
        }
      }
    }
  },
  "headers_other": {},
  "headers_offsets": [
    {
      "name": {
        "Rfc": "content_type"
      },
      "start": 13,
      "end": 45
    }
  ],
  "html_body": [],
  "text_body": [],
  "attachments": [
    1
  ],
  "parts": [
    {
      "Multipart": {
        "content_type": {
          "ContentType": {
            "c_type": "multipart",
            "c_subtype": "mixed",
            "attributes": {
              "boundary": "a"
            }
          }
        }
      }
    },
    {
      "Text": {
        "headers": {
          "content_type": {
            "ContentType": {
              "c_type": "text",
              "c_subtype": "plain",
              "attributes": null
            }
          }
        },
        "body": "body\n--ab\nContent-Type: text/html\n\nbody2\n"
      }
    }
  ],
  "structure": {
    "MultiPart": [
      0,
      [
        {
          "Part": 1
        }
      ]
    ]
  },
  "offset_header": 0,
  "offset_body": 46,
  "offset_end": 167
}