When saving as PPSX (PowerPoint Show), the export always wrote the content type as presentationml.presentation.main+xml (PPTX) instead of presentationml.slideshow.main+xml (PPSX). This caused: - Impress not auto-playing its own PPSX files on re-open - PowerPoint rejecting PPSX files created by Impress - Re-saved PPSX files from other apps losing their autoplay ability Propagate the STARTPRESENTATION filter flag through the export chain (FilterBase -> PowerPointImport -> PowerPointExport) and use it to select the correct slideshow content type, following the same pattern already used for template export. Change-Id: Idcf6222d3855c587d0dabffe3a345c9ef2bfdcf2 Reviewed-on: https://gerrit.libreoffice.org/c/core/+/199884 Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com> Reviewed-by: Miklos Vajna <vmiklos@collabora.com> Signed-off-by: Xisco Fauli <xiscofauli@libreoffice.org> Reviewed-on: https://gerrit.libreoffice.org/c/core/+/204105 Tested-by: Jenkins