I found a new solution, at least creator for manual myself.
Now my preview yukon works again!
Related topics about preview pdf.
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I remarked yesterday, that that the explorer preview didn't more work for PDF-files, although it still worked for text, Word documents and some picture file types.
Fixed in preview pdf in windows 7 under 30 seconds!
Since AI files are embedded inside a PDF file, if I change the file extension to PDF, it previews nicely.