这个密码对我有效。

class DownloadHandler(blobstore_handlers.BlobstoreDownloadHandler):
def get(self):
    filename = self.request.get('file_name')
    key1 = functions.getFileList(filename)
    blob_info = blobstore.BlobInfo.get(blobstore.BlobKey(str(key1.blob)))
    self.send_blob(blob_info,save_as=True)

感谢@Dustin Ingram