code: public LocalFsBlobStoreFile write(String key, boolean create) throws IOException {
          return new LocalFsBlobStoreFile(getKeyDir(key), true, create);
      }
docs: Get an object tied to writing the data.
  @param key the key of the part to write to.
  @return an object that can be used to both write to, but also commit/cancel the operation.
  @throws IOException on any error