This extension offers a number of implementations for a number of built-in functions (suffixed with _ns) that allow GameMaker games to work with files outside of the "sandbox" directories.
It allows application to work with files anywhere where it could by usual rules (which is basically anything not in drive root or other UAC-protected locations).
Paths with non-English characters are supported too (which is something that old versions of GameMaker struggled with).
The extension has been driven mostly obsolete with release of GMS2.2 (with introduction of an option to disable sandbox restrictions through Game Options - Windows/Mac/Linux - General - Disable file system sandbox), but can still be handy for the additional functions.
¹ Most recent version is available on itch.io - GM Marketplace no longer accepts GMS1 uploads