If you open /cgi-bin/uu_default_config.php, you will see a line that reads:

allow_extensions => '(3gp)|(mp4)|(mov)|(asf)|(avi)|(flv)|(wmv)|(mpg)|( mpeg)|(mpeg4)|(xvid)|(divx)|(3gpp)',

There is also a line for disallowing file extensions, in case there's something you want to block.