This is the File I/O Scratch Extension. It also requires a helper app for full functionality, which is located here.
The extension code is in ext.js, and a minified version is in ext.min.js. The documentation/project pages for the Scratch extension and the helper app are also located in this repository.
See the project pages.
The extension initially checks for the helper app by making an OPTIONS request to http://localhost:8080/. It verifies that the response is from the helper app by looking for a X-Is-ScratchX-File-IO-Helper-App header that has a value of yes.
Every request to the helper app it makes is repeated every five seconds until it succeeds. If a request fails and needs to be repeated, the extension status is set to 1,'Waiting for helper app'. Once a repeated request succeeds, the status is reset to 2,'Ready'.