I once used an API where to send a command, you would make a POST where the message body was an x-www-form-urlencoded dictionary where one of the values is a bi

I once used an API where to send a command, you would make a POST where the message body was an x-www-form-urlencoded dictionary where one of the values is a binary string which is a zipped XML document. It was very clearly just a page on the vendor's site where you could manually upload a zip of documents, which they had simply declared to be an API.