Hi Jaheer,
File Attachments that are to be displayed in the Workbench File Attachments Channel are added at the Company level. In Master Data, you will find the Company. In the Company, you can add more File Attachments. This affects every user and should be done by a Administrator of the system. Also, if you do not have the necessary permissions, you will not be able to do this. You will have to work with the individuals responsible for administering the system.
Hope this helps,
Rosemary