I am communicating with a web service that expects a POST parameter and also expect Request body. I have confirmed that such a POST request can be done using a REST Console
You need to make a POST request using multipart-form. Here is the example:
Apache HttpClient making multipart form post
Alternatively, you can make a POST request with the content (parameters and files) encoded using application/x-www-form-urlencoded but it is not recommended when you want to make a POST request with large content, like files.