Content Negotiation for Multipart Response

(I am reposting this question from a w3c registered email because the w3c listing manager has failed to assess my e-mail to this group in one week despite two day commitment. Poor form!) 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