When downloading the offline installer files for Windows there are 2 strange files.
surviving_mars_marsvision_song_contest_evans_249143_32867.sh
surviving_mars_mysteries_resupply_pack_evans_249143_32867.sh
What are these files and how do I use file extension .sh?

to my understanding these files are for Linux, you can check foryoureself next time when you dn on the middle left side System: windows click on check mark under win ,Linux, mac. and you will see these file in there.

Why do they show up when Windows is the selected OS?

asked gog

Lol, those ARE the Linux installers

I am not sure if you selected Linux, or what happened, but if you want to install those under Windows you need the .exe and .bin files. Try reselecting Windows

I did...

I was checking the forums and saw that this game specifically seems to have some bugs with the OS selection (or so I understood). Users from an older Linux distribution seem to be getting the installer from the newer one. Maybe someone in GOG did a cock up... You might want to contact them. I usually get a fast reply when I need them.

Because I use Linux, I have no idea how Galaxy works, as we are not blessed with a version for Linux, but don't you have a "download offline installer" option there or something similar? (again, got no idea, just a wild guess....).

Good luck tho!

I don't use Linux or Galaxy. It's either download through my browser or with GOGrepo. Those files are showing up in both. So I figure they must be flagged as Windows files somewhere.