code: public GitlabHTTPRequestor withAttachment(String key, File file) {
          if (file != null && key != null) {
              attachments.put(key, file);
          }
          return this;
      }
docs: Sets the HTTP Form Post parameters for the request
  Has a fluent api for method chaining
  
  @param key       Form parameter Key
  @param file      File data
  @return this