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Not sure if this is releated but as per this Help Doc [1], it seems there has been some improvements on the technology that renders PowerPoint® file previews. It also mentions that this feature is not GA yet and only some Organizations will have them as of the Spring'14. But by the end of the Summer'14, it should be generally available .
Sorry but for the time sake, I think your best bet will be to resort to PDF versions :-(
[1]: Help Doc (http://docs.releasenotes.salesforce.com/en-us/api/release-notes/rn_general_improved_pptx_file_rendering.htm)
