mail/internal/mail/message.go
func SetCopyFunc(f func(io.Writer) error) FileSetting {
	return func(fi *file) {
		fi.CopyFunc = f
	}
}
func SetCopyFunc(f func(io.Writer) error) FileSetting {
	return func(fi *file) {
		fi.CopyFunc = f
	}
}
// SetCopyFunc is a file setting to replace the function that runs when the
// message is sent. It should copy the content of the file to the io.Writer.
//
// The default copy function opens the file with the given filename, and copy
// its content to the io.Writer.