What is a JEM File Type?
You will probably only encounter a JEM file if you are trying to modify the appearance of the entities in Minecraft with the mod d' Optifine optimization installed. Some of the entities that are modified by Minecraft users include cows, vines, dragons, chickens, books and safes.
If you are looking to bypass an entity, you can place the JEM file in the following directory in the Resource Package.
Goods / mini-crafts / optifin / cem.
Once the JEM file is placed in the directory, it appears in Minecraft as long as the mod d' OptiFine optimization is installed.