That 's odd... I definite uploaded it as a *.schem file using world edit. Minecraft-schematics.com must convert all their uploads into the older format.
However, to answer your question: the only way that I know is to use World Edit to //paste it into a test world (1.13+) and then //copy and //save it again.
The new format will be a schem file.
There are mods/apps that let you go the other way. schem --> schematic, for use with pre-1.13 worlds, but usually not the other way.
Medieval Marketplace on January 18th, 2021 09:06 PM EST