Here's the link to MonsterEffects:
http://dev.bukkit.org/bukkit-plugins/monster-effects/

In this plugin, you can give mobs the ability to "steal" items from a player's inventory. According to the developer, the stolen items goes into the mob's inventory. I want this plugin to have "CONTENTS" also include such "mob inventories."
Instead of attempting to explain what my take on "mob inventories" are (because I know nothing about them), I'll show the developer's perspective on what they are from a "conversation" we had:

Question (from me):
"What exactly does "steal" mean? Monsters don't have an inventory, so where does the "stolen" item go? On the floor?"

Answer (from developer):
"All mobs have got an inventory of 1 slot. This is only visible for a few mobs, like the zombie, because it is able to hold an item. But for skeletons, pig zombies and for all mobs which need an specific item in their hands, the steal effect won't work, because you can't override the existing item.
Answer: The item is placed in their inventory..."

I don't truly know if ALL mobs have a one-slot inventory, and I'm pretty sure zombie pigmen can pick up diamond swords despite the fact that they're already holding gold swords. You would probably know this better than I do.

Regardless of whether it applies to ALL or SOME mobs, like zombies, I'd like "CONTENTS" to include these "mob inventories" wherever it's possible.

This might require a new ticket, but I don't want "CONTENTS" to include the armour a mob is wearing. I'd like some sort of separate parameter for specifying armour drops, like "drop: ARMOR@HEAD". For us who spell the english way, "drop: ARMOUR@HEAD" (notice the "u"). This could be a handy parameter to have.

If you ask me, CONTENTS for a mob should mean their equipment, because that's the most obvious vanilla interpretation. In other words, their armour and weapons.