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 ----
{
  "header": {
    "content_type": {
      "c_type": "multipart",
      "c_subtype": "mixed",
      "attributes": {
        "boundary": ":foo"
      }
    }
  }
}