Hi Eddie - I am @stvwlf who reported the 1.5 bug on Github yesterday. Thank you for fixing that. What I am to say is not specifically about your plugin but an issue I have run into in working with taxonomies on attachment post types, that is, the Media Library.
I am trying to add a taxonomy filter to the top of the Media Library for my custom taxonomy. In the course of doing this I discovered that WordPress is not incrementing the Media count on the taxonomy terms page. I have the term assigned to several images but WordPress is showing a zero in the Media count column (extreme right on taxonomy page). I disabled your plugin to make sure the plugin was not the source of this - it was not the plugin. Then I disabled my custom taxonomy and assigned the standard category taxonomy to the attachment type. I found that when I assign a POST to a category, the media counter increments, as displayed both under Posts and Media Library (since the same taxonom is assigned to both). However when I assign a category to a Media Library image, the Media count does not increment, stays at zero even though on the Media Library page, on the row for that image it shows me in the category column the image is assigned to that category.
I checked in the database and the taxonomy tables are correctly updated with the assignment of that term from that taxonomy to that attachment post id.
The problem is that none of the terms are showing up in the filtering taxonomy dropdown because WordPress sees them as not being assigned to any attachments. I forced the display by overriding Hide Empty and they do show up, but with a post count of zero.
I am asking you about this because you seem very familiar with the workings of the attachment post type and the media library. Have you run into anything like this? Can you think of anything I might be doing wrong?
I appreciate any time you put into answering this.