Hi Vaibhav,
the URL you should be using is the one to add an attachment to a report:
..raylight/v1/documents/<docId/attachments
The error you are getting suggests the body of the request is incorrect. Ensure you have carriage returns in the right places and that the attachment you are sending is added byte for byte into the request.
As shown in the blog about uploading files to the CMS, you can use the Fiddler tool to check out the request body to ensure it is formatted correctly.
Dan