Adding attachment to Siebel

I have a requirement where we need to attach Attachment in Siebel to service request record

we are getting attachment in base64 format from outbound webservice .

can anyone suggest how we can do this .

please note when we import outbound webservice it did not create any IO object.

i have tried below .

call the webservice

get the base64 data in process property

use this process property in Datamap to map it in attachment IO .

i got below error "

Error while mapping file attachment: SBL-EAI-04058: Source expression generated attachment ID '...' but the corresponding attachment cannot be found"