In order to install extension you need the main pack and chosen extension (for example "vanilla_ext.zip"). To make the extension work properly,
you need to drag it above the main pack in the active texture packs section, otherwise it won't work. You can't mix extensions together, except
Technical Font ("tech_font_ext.zip"). Extensions can be found on my Planet Minecraft profile, under this link:

https://www.planetminecraft.com/texture-pack/jellyfish-legacy-compact-and-modern-texture-pack/