Custom objects or fields for attachments

Summary

How can I provide agents with the ability to mark selected attachments?

Content

Hello,

We periodically need to export attachments from OSvC to our eDocument management system. Our agents that manage the attachments in OSvC need to be able to select only certain attachments from each incident for downloading, since there may be attachments that are not needed in the eDocument system.  I didn't see a way to create a custom field on the Attachment table, nor did I see a way to set up a relationship between a custom object and attachments.  So, how do I go about allowing agents to identify attachments without making them use the display name or description fields