What is the value contained in the HTTP Accept-Type header?

The HTTP header Accept-Type is actually called Content-Type or Accept. If it's about Accept, it indicates which media types (MIME types) the client is willing to accept from the server, for example:

Accept: application/json, text/html

This means the client prefers to receive the response in JSON or HTML format. If it's about Content-Type, this header indicates the type of content being sent in the request body, for example:

Content-Type: application/json

Thus, Accept informs the server about which response formats are acceptable, and Content-Type specifies the format of the data being sent.