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

--:foo
--:foo
Content-Type: text/plain
--:foo
Content-Type: text/plain
--:foo
Content-Type: text/html
--:foo--

---- EXPECTED STRUCTURE ----{
  "headers_rfc": {
    "content_type": {
      "ContentType": {
        "c_type": "multipart",
        "c_subtype": "mixed",
        "attributes": {
          "boundary": ":foo"
        }
      }
    }
  },
  "html_body": [],
  "text_body": [],
  "attachments": [],
  "parts": [],
  "structure": {
    "List": []
  },
  "offset_header": 0,
  "offset_body": 48,
  "offset_end": 160
}