I have added the line in .htaccess to my own wiki.
Options -Indexes -MultiViews +FollowSymLinks
Files within the plugin directory can still be accessed.
I have checked other wiki's and they have the file open to the public also.
http://wiki.openwrt.org/lib/plugins/acl/pix/group.png
https://www.zabbix.com/documentation/3.4/lib/plugins/acl/pix/group.png
Should this be a concern?