JsonApi what additional parameters are we talking about?

Question

JSON

I'm parsing the jsonapi.org standard , and almost immediately ran into the following question...
jsonapi.org/format/#content-negotiation-clients

Client Responsibilities
Clients MUST send all JSON API data in request documents with the header Content-Type: application/vnd.api+json without any media type parameters.
Clients that include the JSON API media type in their Accept header MUST specify the media type there at least once without any media type parameters.
Clients MUST ignore any parameters for the application/vnd.api+json media type received in the Content-Type header of response documents.

Namely, the last sentence is not clear, regarding the fact that the Client should ignore other parameters.
It is not entirely clear what parameters are involved? At first, I thought that apart from the Content-Type: application/vnd.api+json header, nothing can be sent to the server, but what about files (multipart/form-data) ?
Found an answer from the author on this topic discuss.jsonapi.org/t/file-uploading-with-multipart/71 and wondered what exactly the parameters are mentioned in the proposal:

Clients MUST ignore any parameters for the application/vnd.api+json media type received in the Content-Type header of response documents.