You will need to modify the LdAppl3.template file that is in your ManagementSuite/ldlogon directory.
Very near the top of that file there is a section for "MultimediaExtensions"
This is the default entry:
MultimediaExtensions=.JPG .JPEG .BMP .MPG .MPEG .MPEG .MP3 .WMA .GIF .MOV .AIFF .M4P
Simply add .AVI to that list. Then go into your console under Reporting and Monitoring-->Manage software list and hit the icon for "Make available to clients"
This will build the LdAppl3.ini with the new information.
When an agent does an inventory scan, it will update this file on the client side and then scan for .AVI files.