soft 404s on attachment tags

Thanks for your question, which has come up before, most recently in these topics:

/attachment_category/ archive pages
Problem with permalinks

As you will see there, WordPress does not support “attachments” in archive pages. The earlier topics suggest several solutions.

I am marking this topic resolved, but please update it if you need any specific help adapting the above solutions to your application. I know it takes some work, but one of the techniques described in the earlier topics can be made to work for you. Thanks for your interest in the plugin.