What api is it using to let you create a file in a local directory? I know about https://developer.mozilla.org/en-US/docs/Web/API/FileSystem but that's for virtualized filesystems
e.g. https://developer.mozilla.org/en-US/docs/Web/API/Window/show...
will give you access to a handle representing a directory on the user's real file system
https://developer.mozilla.org/en-US/docs/Web/API/File_System...