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Microsoft Forefront Security for Exchange Server cannot block .wmv files that have had their file name extensions renamed. This problem occurs because there is no known File Type for the .wmv format.
To resolve this problem, you must install Hotfix Rollup 3 for Microsoft Forefront Security for Exchange Server Service Pack 1. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
951629 Description of Hotfix Rollup 3 for Forefront Security for Exchange Server Service Pack 1
Note Microsoft Forefront Security for Exchange Server Service Pack 1 must be installed before you install this hotfix rollup. After you apply this hotfix, the hotfix adds WMV to the checkfile routine and to the user interface to enable its selection. This is added for all scan jobs in the Forefront user interface.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.