File permissions are independent of Space permissions at the moment. (Sorry, I know this is a bit confusing and we hope to be able to make this part of permissioning more intuitive in a future release.)

You will need to change all of your files to have the 'Public' permission if you'd like them to be world readable, regardless of how you've set your Space permission.

For Space permissions, 'Private' means that the objects in the Space can't be listed by non-authenticated (those without a valid Space key for the account) users. 'Public' means that those files can be listed by anyone, regardless of whether or not they have a valid Spaces key for the account.