@Override
  public void write(Output output, String boundary, String key, Object value)
      throws EncodeException {
    if (value instanceof File[]) {
      val files = (File[]) value;
      for (val file : files) {
        fileWriter.write(output, boundary, key, file);
      }
    } else if (value instanceof Iterable) {
      val iterable = (Iterable<?>) value;
      for (val file : iterable) {
        fileWriter.write(output, boundary, key, file);
      }
    } else {
      throw new IllegalArgumentException();
    }
  }
A writer for multiple files.

@author Artem Labazin
form/src/main/java/feign/form/multipart/ManyFilesWriter.java