I have had no trouble with setting up servers in the past - I have had 10+ servers before that I could play on with different friends. After I set up my server properly, all I had to do was click on the .jar file and it would bring up the Java gui and load everything just as it should. Now, however, when I click on the .jar file, my computer tried to unzip it like a .zip file instead of running it. Nothing that I know of has changed. I did some researching and was finally able to launch the servers using a start.command file, and I even got it to bring up the Java gui when I run said .command file, but it's still not the same and I don't want to add all those extra steps to every server I make.
So essentially the question is: what happened that caused my computer to not be able to immediately run the .jar file like it used to and how to I fix it?