There are no security issues here, and this is not an attack. Special:ListFiles is a special page on the wiki that lists all uploaded images, and from there you can obtain the full path to the image thumbnail.
This could either be some sort of crawler bot, or a person interested in the images on your site. They could be on a dynamic IP, which explains the lack of previous hits, or they could have known somehow that your site is a MediaWiki instance and therefore that Special:ListFiles exists (as it exists on every MediaWiki installation).
Regardless, nothing to worry about.