Manual:$wgAntivirus

Details
Use this setting to select a virus scanner from the list in the $wgAntivirusSetup array. This serves as a key to the $wgAntivirusSetup array, e.g. 'clamav'.

If you set this variable to something other than, and $wgAntivirusSetup is set correctly, every file uploaded will be scanned for viruses.