Help! How do I open downloaded files?

I feel like such a dummy. I used to blame this on my old computer, but clearly it's me.

When I download an attachment, if it's an image, most of the time it appears right on the screen. If it's a document (someones's seed list, for example), it get's stored in a folder.

I can even locate the folder. But when I click on OPEN, it just shows me the e-mail the attachment came with. I know this is terribly basic, but would someone please explain without humiliating me too badly?

Thanks,

Peter