name pngnow.png, type image/png: (right)
 name pngnow.gif, type image/gif: (wrong but at least it's an image type)
 name pngnow.png, type text/plain: (wrong)
 name pngnow.zip, type application/zip: (wrong)
 name pngnow.png, type application/zip: (wrong)
 name pngnow.zip, type image/png: (right)
A browser that supports PNG correctly must display images  and  on this page. Most browsers also display the other images. Whether they should do that is debatable, since they are served with an incorrect MIME type.
Clicking on  and  should display the image in your browser.
Clicking on  should display a mess of plain text in your browser (perhaps only a few characters -- "PNG" should be among them).
Clicking on  and  should behave as if it were a ZIP file: Your browser should probably ask to save to disk or open an unzip application.
What should happen when you click on  is debatable. The browser should either display the PNG image, or indicate a broken (GIF) image.