As you correctly note, the Accept header is used by HTTP clients to tell the server what content types they'll accept. The server will then send back a response, which will include a Content-Type header telling the client what the content type of the returned content actually is.
However, as you may have noticed, HTTP requests can also contain Content-Type headers. Why? Well, think about POST or PUT requests. With those request types, the client is actually sending a bunch of data to the server as part of the request, and the Content-Type header tells the server what the data actually is (and thus determines how the server will parse it).
In particular, for a POST request resulting from an HTML form submission, the Content-Type of the request will (normally) be one of the standard form content types below, as specified by the
enctype attribute on the
application/x-www-form-urlencoded (default, older, simpler, slightly less overhead for small amounts of simple ASCII text, no file upload support)
multipart/form-data (newer, adds support for file uploads, more efficient for large amounts of binary data or non-ASCII text)