Welcome to the forums @kershnerd!
With HTML5 exports, the file system is sandboxed, it cannot access files on the host computer, as noted in the documentation for FileDialog access. This is likely for security reasons, so HTML5 games cannot delete a users files, for example. It could also be a browser restriction, potentially. There is a proposal for adding FileDialog support though linked in the documentation, so its possible this feature could be added in the future.
