After setting up Server 2012 Essentials, I discovered that the native Windows Search indexes .doc, .docx, .ppt, and .xls files, but not .pptx or .xlsx files.
Manually adding Office extensions would treat them as text files, i.e. it would not handle file formats intelligently. For example, I believe it would index the XML tags as well as the content of .xlsx and .pptx files.
Fortunately, I found the following available for download:
Although it does not list Windows 2012 as a supported operating system, I installed it anyway. It added the .xslx and .pptx extensions. After indexing completed and a server reboot, a quick test shows that it is now able to find an .xlsx file that it was missing before.
Update December 5, 2012
Microsoft Partner Support pointed out that Service Pack 1 is available for the Office 2010 Filter Pack: KB2460041. It was a snap to install this from Windows Update.
Support also indicated that there may be an Office 2013 Filter Pack, though the release date is uncertain. There is actually a Filter Central blog, but the last update was two years ago.