I have created an api in amazon api gateway service with s3 proxy, and created a method post to upload a file to s3 using the document. Deployed the API and then using that url i tried to call the api from postman. But i couldn't post the file and it returns an error 'missing authentication token'.
I set authorization as NONE.
Then it returns an Unexpected "<" error.
Ah, okay. S3 only supports POST from an HTML form, which is why you see the error where it is expecting multipart formdata.
The method you need to use is PUT, instead of POST. PUT requires an item path, so you'll need to change the resource path to have a bucket and key, or get those from other places.
I have some more info on how to set this up in upload binary from api gateway to S3 bucket
It sounds like the document you're uploading isn't JSON. By default, API Gateway expects UTF-8 encoded JSON.
What content type are you sending with your Postman request?