    parts: a header with structured metadata and an optional variable-length
        body holding arbitrary data. A serialized header is one or more text lines
        where each line ends in a carriage return followed by linefeed (<code class="computeroutput"><span class="string">"\r\n"</span></code>). An empty line marks the
        end of the header. The first line in the header is called the <span class="emphasis"><em>start-line</em></span>.
        The contents of the start line contents are different 