docs: public Response changeMime(String bucket, String key, String mime)
              throws QiniuException {
          String resource = encodedEntry(bucket, key);
          String encodedMime = UrlSafeBase64.encodeToString(mime);
          String path = String.format("/chgm/%s/mime/%s", resource, encodedMime);
          return rsPost(bucket, path, null);
      }