Passing inputs to RPCs via REST

I have a rpc which can take certain inputs. How to pass the inputs in REST? I tried curl form data to pass the inputs, but did not work.

curl -X POST -iksu admin:admin -H 'Accept: application/vnd.yang.operation+json' http://localhost/api/operations/common-rpc:file-transfer -d 'operation=upload'


{"errors": {"error": [{"error-message": "Unsupported media type: application/x-www-form-urlencoded ; Should be one of: application/vnd.yang.operation+json, application/vnd.yang.operation+xml.", "error-tag": "malformed-message"}]}}

Am I missing something?

See the examples.confd/restconf examples for guidance. The examples uses the curl tool.
Also, I suggest reading RFC 8040: RESTCONF Protocol