Found one file left with this attribute :-)
asterix:SharedSettings pingus$ xattr FRITZ.Box\ Fon\ WLAN\ 7390\ 84.05.22_31.07.12_0844.export
com.apple.metadata:kMDItemDownloadedDate
com.apple.metadata:kMDItemWhereFroms
com.apple.quarantine
asterix:SharedSettings pingus$ xattr -p com.apple.quarantine FRITZ.Box\ Fon\ WLAN\ 7390\ 84.05.22_31.07.12_0844.export
0002;50177eaf;Safari;87F15229-65EB-4643-8C73-D8FC90140161
asterix:SharedSettings pingus$ ls -la FRITZ.Box\ Fon\ WLAN\ 7390\ 84.05.22_31.07.12_0844.export
-rw-r--r--@ 1 pingus staff 205798 31 Jul 08:43 FRITZ.Box Fon WLAN 7390 84.05.22_31.07.12_0844.export
Trying to open this file into TextWrangler produces the well known message about a corrupted file. I found some reports about NFS files having the same issue using Mountain Lion.
Additionally I tried to reproduce this adding new files using my iMac (Lion), but maybe it's just an issue right after the upgrade to mountain lion checking all files under suspicion.