What is your use case? Because on the HR Agent Workspace you have the possibility to just show all attachments on the sidebar. What is the business requirement of having a popup appear with the number of attachments? 

If it really is an important parameter, why not just create a new field 'attachment_count' and put it in the header of the Case, so you always have the info. This explains how you can do that. 

And if that isn't worth it, you could try utilizing this: https://support.servicenow.com/now?id=view_idea&sysparm_idea_id=94b9224097694a10f03d739c1253afff&sys... (found through this question: https://www.servicenow.com/community/developer-articles/verify-attachment-count-in-service-portal-or...)