Adding attachments to index page of http://aed1.branchable.com/ works fine
Create a child page and adding the same attachment gives "bad attachment filename" error.
This was an ikiwiki bug: http://ikiwiki.info/bugs/Unable_to_add_attachments_to_some_pages/
I was able to create a branch of your site and reproduce the problem, which helped me to fix it. Thanks.. Fix is now deployed to Branchable. done --Joey