You basically need to send the file contents as part of the POST body. You may also want to send some metadata about the file. In that case, you should use either Multipart/Form or Multipart/Mixed content types. Basically it allows you to have different kind of data in your POST body.
Here is a very good and simple tutorial to help you learn how to upload a file to a RESTful service:
http://www.mkyong.com/webservices/jax-rs/file-upload-example-in-jersey/