* application/vnd.oasis.opendocument.spreadsheet=ods<br>
     * application/vnd.oasis.opendocument.presentation=odp<br>
     * application/pdf=pdf<br>
     * application/x-fictionbook+xml=fb2<br>
     * application/e-pub+zip=epub<br>
     * application/x-ibooks+zip=ibooks<br>
     * text/plain=txt<br>
     * application/rtf=rtf<br>
     * application/vnd.ms-htmlhelp=chm<br>
     * application/zip=zip<br>

*   Adds `beta` hyperparameter to
    [FTRL](https://www.tensorflow.org/api_docs/python/tf/keras/optimizers/Ftrl)
    optimizer classes (Keras and others) to match
    [FTRL paper](https://research.google.com/pubs/archive/41159.pdf).
*   `Optimizer.__init__` now accepts a `gradient_aggregator` to allow for
    customization of how gradients are aggregated across devices, as well as