Gmail replaced the Flash-based PowerPoint viewer released in 2007 with the same viewer used for PDF files. Some of the features that were available in the old viewer are missing: you can no longer view the presentation as a slideshow or read the titles to easily find a slide.
"This viewer provides a richer set of features than the old "View as slideshow" version: you can zoom in and out, select text to copy and paste, and "print" the presentation to a PDF document. And, unlike the old version, we no longer require you to have a Flash plugin installed on your browser," explains Gmail's blog.
The same viewer is also used for TIFF attachments, which can store one or more images. "The TIFF format is widely supported by image-manipulation applications, by publishing and page layout applications, by scanning, faxing, word processing, optical character recognition applications." As you probably noticed, browsers usually don't support this format, so you need a plug-in to view TIFF files. Google's viewer works by converting the files to PNG images.
Try the new feature by finding Gmail messages that have PPT or TIFF attachments and clicking on "View" next to one of the attachments.