I like your plugin. It has a lot of potential. There is one improvement I would suggest. I currently have 400 media files. I wish to categorize them using your plugin. The thing is that to do that I have to enter into each file and categorize it. This is too time consuming. I suggest to allow batch categorizing of media files. This would make your plugin more usable for sites that already have many media files. Such batch categorization is already built into wordpress regarding posts.
Keep up the good work,