[1] name pngnow.png, type image/png: (right)

[2] name pngnow.gif, type image/gif: (wrong but at least it's an image type)

[3] name pngnow.png, type text/plain: (wrong)

[4] name pngnow.zip, type application/zip: (wrong)

[5] name pngnow.png, type application/zip: (wrong)

[6] name pngnow.zip, type image/png: (right)

A browser that supports PNG correctly must display images [1] and [6] 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 [1] and [6] should display the image in your browser.

Clicking on [3] should display a mess of plain text in your browser (perhaps only a few characters -- "PNG" should be among them).

Clicking on [4] and [5] 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 [2] is debatable. The browser should either display the PNG image, or indicate a broken (GIF) image.