I don't know Guildenstern but what you see here is a "MIME" document, you could either find a mime parser, or you look into other webserver that already implement this:
<https://github.com/guzba/mummy/blob/master/src/mummy/multipart.nim> <https://github.com/dom96/jester/blob/ac9b8541dce64feff9b53b700cab8496c1816651/jester/private/utils.nim#L57>
