I’ve tried asking on the Mediawiki Support Desk but haven’t gotten any help.
EDIT: It looks like the issue with images comes from using Short URLs. After uploading the image through the Upload File section I get a 404 error but if I add /index.php?title= to the url I can visit the uploaded files. Not sure how to fix this though
So basically my issue is mediawiki thinks the links to my files are /File:filename.png but they're actually at index.php?title=File:filename.png
We need more info on your permissions. 755 would mean anyone can read files there, but only the owner can write. If the owner isn't the same user that mediawiki is running as, then uploads won't work.
What info do you need? The error seems to be due to the Short Urls feature. If I go to the Upload File section and upload a file it redirects to the file but gives a 404 message but when I go to the list of uploaded files I can see all the files have actually uploaded.
For a quick test you could set it to 777, if that fixes it, check which user new files are being assigned to, adjust permissions back down accordingly.