This is the javascript mapping, contained in forum.js.map. It allows the browser to show the javascript code formatted just like it's in the source code, making use of the development tools easier, and stack traces in errors actually useful.
If you only have open-source extensions, this is not leaking anything as anyone could go take a look at the source code of those extensions on GitHub. If you have custom private extensions and want to make reading the code slightly more difficult, you can delete the .map files. Though it won't prevent any skilled developer from reverse-engineering the code.
This does not leak any server-side (PHP) code, and doesn't allow anyone to modify any file on your server.
