Also correct processing "text/html" and "image/png" mime types
to fix slow select in Calc:
if both "text/html" and "image/png" mime types were present,
"image/png" data was requested, but it was processed as if it was
"text/html" data.

Change-Id: Ie192fff06cd9de28f466547dac32343620fe2730
Reviewed-on: https://gerrit.libreoffice.org/66576
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>