The official file extension for reStructuredText files is .txt, but this is a stupid standard that should be ignored.
2.4 What's the standard filename extension for a reStructuredText file?
It's ".txt". Some people would like to use ".rest" or ".rst" or ".restx", but why bother? ReStructuredText source files are meant to be readable as plaintext, and most operating systems already associate ".txt" with text files. Using a specialized filename extension would require that users alter their OS settings, which is something that many users will not be willing or able to do.