'Opens a Stream from which the application can read the contents of
the stored file specified by file_id.
For example::
my_db = MongoClient().test
fs = GridFSBucket(my_db)
# get _id of file to read.
file_id = fs.upload_from_stream("test_file", "data I want to store!")
grid_out = fs.open_download_stream(file_id)
contents = grid_out.read()
Returns an instance of :class:`~gridfs.grid_file.GridOut`.
Raises :exc:`~gridfs.errors.NoFile` if no file with file_id exists.
:Parameters:
- `file_id`: The _id of the file to be downloaded.'

def open_download_stream(self, file_id):

    gout = GridOut(self._collection, file_id)
    gout._ensure_file()
    return gout
