ECMS-6558: No preview of Powerpoint (.ppt) or Excel (.xls) files

Fix description:
* Update MIME type for Powerpoint (.ppt, .pps) and Excel (.xls, .xlt) in FileFilter and ThumbnailFilter
* Update MIME type for Powerpoint (.pps) and Excel (.xlt) in mimetypes.properties
Merge commit 'dc1c12aa8b13bfd2dd0ab7b08662bb38cbc64b90' into integration/4.2.x-translation