In bug 8161 we replaced the deprecated Lib/cgi.py with our own function for parsing POST requests, called parse_post_request, which only handles requests where the request body is URL-encoded.
It is not a complete replacement for the cgi.FieldStorage class we used previously, which can handle arbitrary Content-Types. Most importantly, cgi.FieldStorage handles the multipart Content-Type, which is used when you e.g. want to allow users to upload large files via an HTML-form.
We should therefore add handling for multipart requests  to our parse_post_request in case this is needed in the future.
 Keep in mind that a multipart request can contain arbitrary Content-Types in the request body.