Content Negotiation for Multipart Response

I have seen similar questions asked elsewhere, but I cannot seem to find an authoritative answer. Neither has reading the relevant RFCs helped.

Here is my use-case. I would like to design a resource whereupon a GET request, the client receives the representation of the resource followed by a stream of notifications about changes to that resource. Now, I understand the response will contain the header `Content-Type: multipart/mixed; boundary=<random string>` with each content piece then having their own `Content-Type: something/else` specified. Now my question is how does the client, say though the Accept header (or another means), specify the MIME type for the individual component messages, i.e. specify `something/else`.. In particular, what if a client wants separate Content-Types for the primary representation and the subsequent notifications?

Thanks in advance,
Rahul


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