Working as designed.
Release : 17.2
Component : SDM KNOWLEDGE MANAGEMENT
The role to be able to delete a file has to be admin.
We have this condition "<PDM_IF "$args.KEEP.ADMIN" == "1">" ( i.e. list_attmnt.htmpl)
So, as per the current design only roles that are admins are allowed to delete attachments (same for images).
You can try to customize (out of the scope of support ) but not recommended due security reasons and it could be that elsewhere in the code the admin privilege is verified again.