OpenFL running in a web browser provides basic file reading and writing capabilities, via the FileReference class. For security reasons, when running in a web browser, the user must always grant permission before you can read or write a file.
OpenFL on native platforms provides more complete access to the file system of the host computer than is available on the web. Using the OpenFL file system API, you can access and manage directories and files, create directories and files, write data to files, and so on.