Export some neccessary types to read info

[Guest-615695028]

We are expected to see exported types and fields to create MessageSetting, PartSetting, FileSetting, etc..

type Message struct {
	header      header
	parts       []*part
	attachments []*file
	embedded    []*file
	charset     string
	encoding    Encoding
	hEncoder    mimeEncoder
	buf         bytes.Buffer
}

type header map[string][]string

type part struct {
	contentType string
	copier      func(io.Writer) error
	encoding    Encoding
}

type file struct {
	Name     string
	Header   map[string][]string
	CopyFunc func(w io.Writer) error
}

→

type Message struct {
	Header      Header
	Parts       []*Part
	Attachments []*File
	Embedded    []*File
	Charset     string
	Encoding    Encoding
	Encoder     mime.WordEncoder
	Buf         bytes.Buffer
}

type Header map[string][]string

type Part struct {
	ContentType string
	Copier      func(io.Writer) error
	Encoding    Encoding
}

type File struct {
	Name     string
	Header   Header
	CopyFunc func(w io.Writer) error
}

[Guest-615695028]

I tried to modify them myself, but what is the effect to the coupled components?