How are parameters sent in an HTTP POST request?

The values are sent in the request body, in the format that the content type specifies.

Usually the content type is application/x-www-form-urlencoded, so the request body uses the same format as the query string:

parameter=value&also=another

When you use a file upload in the form, you use the multipart/form-data encoding instead, which has a different format. It’s more complicated, but you usually don’t need to care what it looks like, so I won’t show an example, but it can be good to know that it exists.