May be a tall order!
This isn't sure fire, it will take a bit of development, but here's where i would start.
- Create a new Module
- Create a new Class to house the new page attachment meta data, that has an int field that is a Binding reference to the CMS Page Attachments
Then, in the Module > User Interface, see if you can add a new interface to the Media Dialog.
- You may be able to add a child to the Attachments tab to house your logic, but possibly not as the Attachment tab isn't really a tab-display template.
- If the above doesn't work, you'll need to create a interface next to the Attachments tab, and then create your own interface to read the current page's attachments, list them and allow you to add additional attributes which will fill in your new class
Once you have all this, you'll need to either clone and modify the existing tools that pull in the page attachments, or create your own that will utilize the expanded modules.
All the above can be found in the documentation, but again it's a pretty large task!