It will be more difficult to get to the individual attachment by the collection, unless you use the SDK or some ToolKit (like Java or .NET) that will allow you to examine and iterate over the collection objects.
So my suggestion is this:
1. You can use the query I sent you earlier to find all user stories in backlog that have attachments.
2. Then, for any such user story (which you know the Name) you can use the Attachment endpoint and filter by the Artifact field such as: (Artifact.Name = "Your User Story Name") . This will return all individual attachments of that particular user story where you can fetch the Name, Description, ContentType etc..
Check out the 'Attachment' endpoint and see it's field called: Artifact. That will probably be the easiest way to get it all done.
So, in step 1 you find all your user stories, then one-by-one you query the Attachment endpoint for each such story and get the info on its attachments.
Let me know if helped.
Sagi