Uses the same approach as DOCX import to preserve the ProgID; it would
be much better if the MediaType of the stream were preserved instead and
the other things derived from that, but this here was rather quick to do...

This makes the round-tripping of OOXML OLEs in PPTX work again, which
was broken by an earlier commit.

Change-Id: Ic7d0362f0c14bf0e522185713666bcd58db2cf64