def file_urls_mutation(dataset_id, snapshot_tag, file_urls):
    """
    Return the OpenNeuro mutation to update the file urls of a snapshot filetree
    """
    file_update = {
        'datasetId': dataset_id,
        'tag': snapshot_tag,
        'files': file_urls
    }
    return {
        'query': 'mutation ($files: FileUrls!) { updateSnapshotFileUrls(fileUrls: $files)}',
        'variables':
        {
            'files': file_update
        }
    }