The bug
"Don't forget to backup this world before you load it in this snapshot."
The saved file names are not attaching the .zip, they are functional zip files once the .zip is added back to their names.
Code analysis
LevelStorage.backupLevel(final String name) now uses WorldNameProvider.transformWorldName(path, string) to determine the world name, but does not append ".zip" anymore.
Linked issues
is duplicated by 2
Comments 3
Confirmed in 19w12b.
I was able to just add ".zip" to the file name and open it without any problems. I could also use 7-zip to open the file without an extension. If your backup is corrupted, that's probably an unrelated issue.
Same thing happened to me. How do you turn it back into a functional zip file though? I added the .zip manually but it gives me a warning that the file is damaged or cannot be read.