List of System.IO.Packaging ContentType strings?

I'm updating some older code that uses System.IO.Packaging to programmatically create Excel files. It ultimately calls CreatePackage on various bits of internal state to build out the documents. CreatePackage takes a ContentType parameter, and the existing code contains a list of constants like:

Const cxl07WorksheetContentType = "application/vnd...."

I'm trying to add support for PivotTables, which require a PivotCache. I cannot find the appropriate ContentType. I thought I might be able to discern it from within the file, looking in _Rels for instance, but these are always in URI form and bear no obvious relationship to these constants.

So...

1. is this even required? I passed Nothing and "" but that did not work.
2. does anyone know where these might be defined? I looked on the mime database, and the MS website, but nothing came up on either for "pivot"
3. where are these even used? they do not appear in the resulting Package as far as I can see.