Hi Chetan
1) The work item to which you are adding the attachment to is the work item ID of the Background step or the work item ID of the Workflow?
2) Is the method Instance dependent ? If yes, are you binding the whole of the object instance from the WF to the _WI_OBJECT_ID of the background task? If the method is Instance independent, are you binding the work item id from the TASK to the Method?
3) If the Work item ID you are passing is of the Background step, are you binding back the _ATTACH_OBJECTS of the Task back to the Workflow?
Regards,
Modak