There is a resource attachment and a description attachment:I'm struggling this morning. I also tried 7zip and that was not allowed.
The file uploaded to the resource "Type" is THE file for the resource listing. You can give it versions and it gets special graphical positioning for downloading etc. The description field will allow you to attach files as well, however it is limited to images only as the description isn't meant for the download of resources.
I've changed the phrase to say "Upload Resource" so now it should be less confusing.