Some users were having trouble getting GTK+ dialogs to work on Mac and Linux, so I decided to create a quick solution to this problem, based on "Tiny File Dialogs" from SourceForge.net, which was licensed under zlib.
Note: currently only works on Mac and Linux. Will update the code soon, so that it will work on Windows as well, out of the box.



As you can see from the screenshots, this extension is also available for GameMaker Studio. It works the same in both.
Here's the progress I've made with the Windows version; (I can't publish it until a certain bug is fixed with MinGW):

What you see in the Windows screenshots above, was actually made with Microsoft's compiler, which is why it works in GMStudio but not ENIGMA yet.
All of pre-Studio GameMaker and GameMaker Studio's dialog functions are mirrored with the *_tfd() suffix.
Download here:
[link removed] Note: This ENIGMA extension is now it's own Widget System and is shipped with ENIGMA.
Extract that zip under this directory:
"[YourEnigmaFolder]/enigma-dev/ENIGMAsystem/SHELL/Universal_System/Extensions/"
Comes with an editable GM81 file and two Linux demo executables, (one for 32-bit and one for 64-bit).
To install all of your game's dependencies on Linux, along with the Linux dependencies that are needed to use this extension, use this terminal command:
sudo apt-get install g++ zlib1g-dev libglew-dev libglu1-mesa-dev libalure-dev libvorbisfile3 libvorbis-dev libdumb1-dev libbox2d-dev zenity
The only dependencies this modified version of Tiny File Dialogs uses are zenity and GTK, as far as I know.
Cheers.
Samuel