Mimetype is calculated based on the file's content. If ``_mimetype``
        attribute is available, it will be returned (backends which store
        mimetypes or can easily recognize them, should set this private
        attribute to indicate that type should *NOT* be calculated).

def get_mimetype(self):
        """
        Mimetype is calculated based on the file's content. If ``_mimetype``
        attribute is available, it will be returned (backends which store
        mimetypes or can easily recognize them, should set this private
        attribute to indicate that type should *NOT* be calculated).
        """
        if hasattr(self, '_mimetype'):
            if (isinstance(self._mimetype, (tuple, list,)) and
                len(self._mimetype) == 2):
                return self._mimetype
            else:
                raise NodeError('given _mimetype attribute must be an 2 '
                               'element list or tuple')

        mtype, encoding = mimetypes.guess_type(self.name)

        if mtype is None:
            if self.is_binary:
                mtype = 'application/octet-stream'
                encoding = None
            else:
                mtype = 'text/plain'
                encoding = None
        return mtype, encoding