Why won't my icons paste?

When I copy and paste the new app icons I want, they won’t paste and change the icon. I have done this once before and it worked fine. The icons I have downloaded are from interfacelift. I have noticed that when I look at the info, the icons don’t have any extention (.png .ico .jpg), and Preview can’t be set as the default app to open them because the info says they’re documents.