Ashish,
Did you try to check our Attach Document landing page section yet?
Troubleshooting: SAP Work Manager 6.0 / 6.1 / 6.2 Attach Document issue
We update this landing page with any known new unique issue. Most customers can get it working after looking at our checklist. We have added requirements like does the user have authentication rights to download documents and others.
Best Regards,
Mark Pe
SAP Platinum Support Engineer