There is an experimental beta filesystem in MakeCode, you can find it by going to the "Add packages" menu and searching for "files":



You can find the documentation here: https://makecode.microbit.org/pkg/microsoft/pxt-filesystem


Please note that this MakeCode feature is not quite finished or ready for general use, and so it has been clearly marked as "beta". It is possible that it might not work exactly as expected, and it might contain some bugs, so please proceed with caution.