I have a weird thing here. Fresh Tiki 8.3 install and I started uploading files for the wiki. JPG works fine but everytime I upload a png (zip or individual) it does not generate a thumbnail and when I try to add it to the wikipage it says "not an image file".
Edit: Hm wait, the picture is actually visible at the wikipedia page when I don't use the &thumbnail in the source. But the thumbnail (mouse over) is still missing. How do I get my thumbnails?