Content-type is set by the first writer added to the workspace.

For example:

If I add a KML writer, then second add a PDF writer. The Data Streaming service will send out a content-type of: application/vnd.google-earth.kmz .
If the PDF writer was added first and the KML writer second, then the content-type would be: application/pdf

I've filed an enhancement request to have this added to the Data Streaming service documentation.

https://docs.safe.com/fme/html/FME_Server_Document...