'Check if a file exists in this instance of :class:`GridFS`.
The file to check for can be specified by the value of its
``_id`` key, or by passing in a query document. A query
document can be passed in as dictionary, or by using keyword
arguments. Thus, the following three calls are equivalent:
>>> fs.exists(file_id)
>>> fs.exists({"_id": file_id})
>>> fs.exists(_id=file_id)
As are the following two calls:
>>> fs.exists({"filename": "mike.txt"})
>>> fs.exists(filename="mike.txt")
And the following two:
>>> fs.exists({"foo": {"$gt": 12}})
>>> fs.exists(foo={"$gt": 12})
Returns ``True`` if a matching file exists, ``False``
otherwise. Calls to :meth:`exists` will not automatically
create appropriate indexes; application developers should be
sure to create indexes if needed and as appropriate.
:Parameters:
- `document_or_id` (optional): query document, or _id of the
document to check for
- `**kwargs` (optional): keyword arguments are used as a
query document, if they\'re present.'

def exists(self, document_or_id=None, **kwargs):

    if kwargs:
        return (self.__files.find_one(kwargs, ['_id']) is not None)
    return (self.__files.find_one(document_or_id, ['_id']) is not None)
