code: public BoxFileUploadSession.Info createUploadSession(long fileSize) {
          URL url = UPLOAD_SESSION_URL_TEMPLATE.build(this.getAPI().getBaseUploadURL(), this.getID());
  
          BoxJSONRequest request = new BoxJSONRequest(this.getAPI(), url, "POST");
          request.addHeader("Content-Type", "application/json");
  
          JsonObject body = new JsonObject();
          body.add("file_size", fileSize);
          request.setBody(body.toString());
  
          BoxJSONResponse response = (BoxJSONResponse) request.send();
          JsonObject jsonObject = JsonObject.readFrom(response.getJSON());
  
          String sessionId = jsonObject.get("id").asString();
          BoxFileUploadSession session = new BoxFileUploadSession(this.getAPI(), sessionId);
          return session.new Info(jsonObject);
      }
docs: Creates an upload session to create a new version of a file in chunks.
  This will first verify that the version can be created and then open a session for uploading pieces of the file.
  @param fileSize the size of the file that will be uploaded.
  @return the created upload session instance.