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

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

--a
Content-Type: text/plain

body

---- EXPECTED STRUCTURE ----
{
  "headers": {
    "values": {
      "content_type": {
        "ContentType": {
          "c_type": "multipart",
          "c_subtype": "mixed",
          "attributes": {
            "boundary": "a"
          }
        }
      }
    },
    "offsets": [
      {
        "name": {
          "Rfc": "content_type"
        },
        "start": 13,
        "end": 44
      }
    ]
  },
  "attachments": [
    {
      "Attachment": 1
    }
  ],
  "parts": [
    {
      "Multipart": {
        "values": {
          "content_type": {
            "ContentType": {
              "c_type": "multipart",
              "c_subtype": "mixed",
              "attributes": {
                "boundary": "a"
              }
            }
          }
        },
        "offsets": [
          {
            "name": {
              "Rfc": "content_type"
            },
            "start": 62,
            "end": 93
          }
        ]
      }
    },
    {
      "Text": {
        "headers": {
          "values": {
            "content_type": {
              "ContentType": {
                "c_type": "text",
                "c_subtype": "plain"
              }
            }
          },
          "offsets": [
            {
              "name": {
                "Rfc": "content_type"
              },
              "start": 111,
              "end": 123
            }
          ]
        },
        "contents": "body\n",
        "is_html": false
      }
    }
  ],
  "structure": {
    "MultiPart": [
      0,
      [
        {
          "Part": 1
        }
      ]
    ]
  },
  "offset_header": 0,
  "offset_body": 45,
  "offset_end": 129
}