Upon viewing the inventory of a tamed mule/donkey, there is not a spot for chest placement (the feature for mobile item storage). This was tested in both creative and survival, and I restarted my system three times. (Also tested while riding the animal and while not, this bug is still present.)
MC-54623
Mules and Donkeys Do Not Have An Option for Chest Inventory
Resolved
Invalid
0
It is not in the UI, you simply right click on the mule or donkey with the chest and it will apply it.