RFC 1867 single file
("field1", None, "text/plain", None, "Joe Blow")
(
  "pics",
  Some("file1.txt"),
  "text/plain",
  None,
  "... contents of file1.txt ...",
)
RFC 7578 transfer encoding stays raw
(
  "field1",
  None,
  "text/plain;charset=UTF-8",
  Some("quoted-printable"),
  "Joe owes =E2=82=AC100.",
)
RFC 7578 repeated field names
("files", Some("one.txt"), "text/plain", None, "one")
("files", Some("two.txt"), "application/octet-stream", None, "two")
RFC 7578 charset field remains a part
("_charset_", None, "text/plain", None, "UTF-8")
("message", None, "text/plain", None, "月亮🌙")
