The .jar extension stands for Java Archive. This is the executable package that Minecraft’s Java engine reads. If your file does not end in .jar , your operating system will not recognize it as a mod.
A security analysis of a fake Meteor Client file in the wild, named meteor-client-1.21.10-54.jar , showed it had 76 indicators of malicious behavior, including attempts to modify system security settings and execute native Windows tools in suspicious ways. Another analysis of meteor-client-0.5.8.jar flagged it with the generic identifier Malware.U.Gen.tr , which indicates it exhibits "Trojan horse characteristics". This is the kind of risk you take when downloading from unverified sources. file name meteorclientmodfabric121jar new