Ever want to be able to drag files into the GameMaker window by way of loading them, rather than having to go through get_open_filename()? This is for you!
You will need to disable the filesystem sandbox (Game Options > Windows) to be able to actually access the files you drag into the window. This should technically be usable in GMS1, but the filesystem will make using it a bit of a pain - unless you're also using an extension to get around that, as well.
This extension is a DLL which uses the Windows API, and therefore only works on Windows targets.