While I think small extensions sound cool, they're relics of a time where there was reasons to demand smaller extensions, actual name string size limitations, path length limitations, smaller monitors, being stuck in terminals with only 80 characters width, and an abundance of choice that wouldn't conflict with anything else.
Aside from looking unconventional, is there any reason to not use something like ".joplin_plugin", it's absolutely clear at a glance what it is. (Edit: my friend provided this link that shows this isn't too obscure like some readers might think, https://fileinfo.com/filetypes/plugin, long extensions like this are used by other big tools like Adobe, Apple's XCode, Sublime, etc)
Otherwise is it worth not actually just using .tar? A lot of software uses custom extensions for otherwise typically compressed folders, .deb .jar .epub etc, and it makes it a hinderance to tell someone how they could for example normally right click them and open them in an archive manager tool, but instead there's extra steps of figuring out what kind of file it is, and it kind of just imposes some artificial confusion that there's more going on than there actually is and I don't see the point, if it walks like a duck and quacks like a duck, it's a tarfile