This is because, SharePoint server 2007 search crawler doesn't recognize Office 2010 file types (such as .docx, .pptx, .xlsx,etc).
Solution: Download and Install Office 2010 Filter pack from http://www.microsoft.com/en-us/download/details.aspx?id=17062 to the Index server.
This filter pack installs few dlls and fixes the above issue. If you happen to see this error on PDF documents, make sure you have PDF IFilters installed and configured.
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