At this time, Conditional Widgets doesn’t provide any logic for handling widgets on Attachment Pages. I’d suggest something like Widget Logic instead – especially given your advanced knowledge. Conditional Widgets is largely intended to be for “non-power” users.
This problem reared its head again and I initially forgot I put in a post on this forum 🙂
I went in and actually added support for the attachment pages. Only needed like a half dozen lines of code. I updated logic.php, form.php and cets_conditional_widgets.php Any of the new code I put in has a NEW beside it so it’ll be easy to find. The updated files are here if you wish to add them to a future release:
http://jlmconsulting.ca/wp-content/uploads/2017/01/ConditionalWidgetsAttachmentUpdate.zip
I’m currently using WordPress 4.7.2 and it works properly.
If you have any questions or need any help, please feel free to give me a shout! Thanks again!