feat(preview): Add thumbnail providers for RTF and MSPowerPoint

[timar]

RTF (application/rtf) and MSPowerPoint (application/vnd.ms-powerpoint) were missing from the preview providers, so no thumbnails were generated for these file types in the Files view.

• Resolves: #
• Target version: main

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required

[github-actions]

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