Nice little plugin this, not actually any use to me now (had plans but i've come up with another solution), but would have been ideal for a couple of users who posted support questions previously asking for something along these lines (attachment tagging).
However, i'm actually posting to report a slight oversight in the plugin code.
The plugin is not correctly using translatable strings/values, nor does it register a plugin text domain (required for translation), so the usage of
__(), etc.. is currently pointless.
If you'd like some help making the plugin translation ready feel free to just let me know by way of response here..
If you'd prefer to deal with preparing the plugin yourself, i've written a very basic(not in-depth, just covers the key points) article on how to make plugins translation ready, which you can find here.
Great job on the plugin though, does exactly what it says... :)