Comment 2 for bug 677636

I have no idea how the code is written, but it's better to check for mime-type rather than file extension. You can create a gif-file but name it 'test.png'. It's still a .gif-file even though the extension "tells you" it's a gif-file.

I don't know which language the site is written in, but in php it's quite simple to check with fileinfo().