Not sure if this is old news or not, but regarding people who are having trouble with opening images in the "SPRT" folder. Truth is they're not images, they are .json files (so are the files in many of the other folders). They contain data for animation, coordinates, etc but not the images themselves, which are instead in the "TXTR" folder (and they're all jumbled, so anyone who makes a script that fixes this - or gives a source code file - would be most appreciated).
Here is an example of what the code in one of the .json files looks like (viewable in firefox for sure): http://biancagames.com/bullet_enemy.json
Although the quickbms script exported the "SPRT" files without an extension, I found another decompiler: https://gitlab.com/PoroCYon/Altar.NET/tags
that exported them as .json files. Just so everyone knows, I think this decompiler also can decompile readable gml code.
I hope this helps someone! ♥