docs: private void appendPart(String formFieldName, Resource resource, OutputStream out) {
  		if (resource instanceof MultiResource) {
  			// 多资源
  			for (Resource subResource : (MultiResource) resource) {
  				appendPart(formFieldName, subResource, out);
  			}
  		} else {
  			// 普通资源
  			final StringBuilder builder = StrUtil.builder().append("--").append(BOUNDARY).append(StrUtil.CRLF);
  			final String fileName = resource.getName();
  			builder.append(StrUtil.format(CONTENT_DISPOSITION_FILE_TEMPLATE, formFieldName, ObjectUtil.defaultIfNull(fileName, formFieldName)));
  			builder.append(StrUtil.format(CONTENT_TYPE_FILE_TEMPLATE, HttpUtil.getMimeType(fileName)));
  			IoUtil.write(out, this.charset, false, builder);
  			InputStream in = null;
  			try {
  				in = resource.getStream();
  				IoUtil.copy(in, out);
  			} finally {
  				IoUtil.close(in);
  			}
  			IoUtil.write(out, this.charset, false, StrUtil.CRLF);
  		}
  
  	}