E/Multipart RFC 2822 formatted data into file.

        file must be a file-like object, not a path.  It needs to be opened in
        binary mode.

        If a value is a string or a CompressedValue, it is written directly.
        Otherwise it must be a tuple containing the source file and an optional
        boolean value (in that order); the first argument can be a file name or
        a file-like object, which will be read and its content will become the
        value of this key.  The file will be gzip compressed, unless the key
        already ends in .gz.

        attach_treshold specifies the maximum number of lines for a value to be
        included into the first inline text part. All bigger values (as well as
        all non-ASCII ones) will become an attachment, as well as text
        values bigger than 1 kB.

        Extra MIME preamble headers can be specified, too, as a dictionary.

        skip_keys is a set/list specifying keys which are filtered out and not
        written to the destination file.

        priority_fields is a set/list specifying the order in which keys should
        appear in the destination file.
        Ú