Mahendra,
Check the pictures in the middle of this document tied to file sizes.
Troubleshooting: SAP Work Manager 6.0 / 6.1 / 6.2 Attach Document issue
You need to specify file sizes (look at the picture and the one with the red mark). Sometimes the default set is 0. Check yours and increase it accordingly.
Hope this helps.
Mark Pe
SAP Platinum Support Engineer