Hi Manjula
Attachments are not supported in SAP's REST adapter. Please refer to the following SAP Online library link.
REST Adapter Overview - Advanced Adapter Engine - SAP Library
As suggested by Maheswarareddy, you can opt for the SOAP adapter instead as it supports attachment in the receiver channel and it behaves like an HTTP POST. You can check "Do not use SOAP envelope" so that the payload is transferred to the receiver system without the SOAP envelope.
If you still can't resolve it, please elaborate further on your scenario especially what is the receiver system and its constraints/requirements.
Rgds
Eng Swee