Fix file format

[zac-yang]

Hey @danielfrg, I discovered an issue when copy/pasting binary files with s3contents, where the new file ends up being corrupted. Seems like the root cause is that GenericContentsManager._file_model_from_path() does not populate model["format"] correctly for these files. This propagates through _get_file() and get(), such that when save() and _save_file() is called the incorrect default text format is getting passed through to fs.write().

The changes I made broadly follow the notebook base implementation, see:

• https://github.com/jupyter/notebook/blob/f354740e57f206d67bfb077a9f23bb8d22b6b311/notebook/services/contents/filemanager.py#L344-L374
• https://github.com/jupyter/notebook/blob/f354740e57f206d67bfb077a9f23bb8d22b6b311/notebook/services/contents/fileio.py#L293-L320

[danielfrg]

Thanks! I will do a small release today if i get some free time.