EXTRA.DAT files is release when there is new and could be critical to your environment, When new malware is discovered and extra detection is required to detect and prevent against the threat.
EXTRA.DAT files contain information that is used by VirusScan and other McAfee products to detect new malware.
EXTRA.DAT file may be made available until the normal DAT update is released.
Note: EXTRA.DAT files are valid for 30 days, after which time they will become disabled.
How to update EXTRA.DAT in ePO server
Go to Menu -> Master Repository
Click “Check in Package”
Make sure you enable the EXTRA.DAT on your update tasks.