Get the files or file contents of a file for an entity.

        If all files are requested, a dictionary of filenames and urls for the
        files in the archive are returned.

        If filename is provided, the url of just that file is returned, if it
        exists.

        If filename is provided and read_file is true, the *contents* of the
        file are returned, if the file exists.

        @param entity_id The id of the entity to get files for
        @param manifest The manifest of files for the entity. Providing this
            reduces queries; if not provided, the manifest is looked up in the
            charmstore.
        @param filename The name of the file in the archive to get.
        @param read_file Whether to get the url for the file or the file
            contents.
        @param channel Optional channel name.

def files(self, entity_id, manifest=None, filename=None,
              read_file=False, channel=None):
        '''
        Get the files or file contents of a file for an entity.

        If all files are requested, a dictionary of filenames and urls for the
        files in the archive are returned.

        If filename is provided, the url of just that file is returned, if it
        exists.

        If filename is provided and read_file is true, the *contents* of the
        file are returned, if the file exists.

        @param entity_id The id of the entity to get files for
        @param manifest The manifest of files for the entity. Providing this
            reduces queries; if not provided, the manifest is looked up in the
            charmstore.
        @param filename The name of the file in the archive to get.
        @param read_file Whether to get the url for the file or the file
            contents.
        @param channel Optional channel name.
        '''
        if manifest is None:
            manifest_url = '{}/{}/meta/manifest'.format(self.url,
                                                        _get_path(entity_id))
            manifest_url = _add_channel(manifest_url, channel)
            manifest = self._get(manifest_url)
            manifest = manifest.json()
        files = {}
        for f in manifest:
            manifest_name = f['Name']
            file_url = self.file_url(_get_path(entity_id), manifest_name,
                                     channel=channel)
            files[manifest_name] = file_url

        if filename:
            file_url = files.get(filename, None)
            if file_url is None:
                raise EntityNotFound(entity_id, filename)
            if read_file:
                data = self._get(file_url)
                return data.text
            else:
                return file_url
        else:
            return files