Apple Security: "Safe" Files
Things that Apple’s OS X 10.8.3 update patched — which may allow for arbitrary code execution — that are also on “safe” file type list used by Mac browsers: ImageIO (picture); IOAcceleratorFamily (picture); PDFKit (PDF); QuickTime (movie).
Open “safe” files after downloading… on by default.
You may wish to consider unchecking that particular box.
On 15/03/13 At 02:22 PM
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Story added 15. March 2013, content source with full text you can find at link above.