I've added a one line change to the attachment checkout source code that should resolve this issue. Thanks to some recent updates done by Ray Barham the plugin actually compiles now, and works on Confluence 2.10.3.
I've contacted the recent author to ask/prompt them to take ownership of the plugin. If they have no interest I'll post a build on the attachment checkout page like they have done with the update. (The new plugins.atlassian.com website for managing repository plugins makes it impossible to release a plugin to the repo if you aren't the author unfortunately.)
Further investigation shows that this is an issue with the checkout plugin, it can't be fixed on the Gliffy side.
http://developer.atlassian.com/jira/browse/CHKOUT-29