Remote file identifier; may be empty. Can be used by the current user across application restarts or even from other devices. Uniquely identifies a file, but a file can have a lot of different valid identifiers. If the identifier starts with " http://" or "https://", it represents the HTTP URL of the file. TDLib is currently unable to download files if only their URL is known. If downloadFile/addFileToDownloads is called on such a file or if it is sent to a secret chat, TDLib starts a file generation process by sending updateFileGenerationStart to the application with the HTTP URL in the original_path and "#url#" as the conversion string. Application must generate the file by downloading it to the specified location