code: public ClientMediaEntry createMediaEntry(final String title, final String slug, final String contentType, final byte[] bytes) throws ProponoException {
          if (!isWritable()) {
              throw new ProponoException("Collection is not writable");
          }
          return new ClientMediaEntry(service, this, title, slug, contentType, bytes);
      }
docs: Create new media entry assocaited with collection, but do not save. server. Depending on the
  Atom server, you may or may not be able to persist the properties of the entry that is
  returned.
  
  @param title Title to used for uploaded file.
  @param slug String to be used in file-name of stored file
  @param contentType MIME content-type of file.
  @param bytes Data to be uploaded as byte array.
  @throws ProponoException if collecton is not writable