In my opinion, I don't think we can solve this problem according to add more items in MIME type of IIS.
We can add a new "fileExtension" in MIME type and set its "mimeType" to allow IIS recognize this kind of file extension. But we can't add a new "mimeType" to ask IIS recognize this kind of "mimeType". In other words, we can input anything(eg .aa .bb .cc) for the value of "fileExtension", but we can't use a nonexistent type for the value of "mimeType".
<mimeMap fileExtension="anything(eg .aa .bb .cc)" mimeType="we can just use common MIME types" />
The operation "append user-agent to content-type" which you mentioned in your question is strange. I think you'd better set the user-agent in the header of the http request in your code instead of append it to content-type.
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