code: public AppendObjectResponse appendObject(String bucketName, String key, File file) {
          return this.appendObject(new AppendObjectRequest(bucketName, key, file));
      }
docs: Uploads the specified appendable file to Bos under the specified bucket and key name.
  
  @param bucketName The name of an existing bucket, to which you have Write permission.
  @param key The key under which to store the specified file.
  @param file The appendable file containing the data to be uploaded to Bos.
  @return An AppendObjectResponse object containing the information returned by Bos for the newly created object.