I have MDF in our wardrobe 132*50*12, it has support on the short sides as well as on the long side at the back... holds really well!!
You can also attach a strip or a strip of the MDF board at the back edge of the shelf, depending on which material you have on the back of the wardrobe. Make the shelf 16mm shorter in depth and then glue/nail a strip that is 16mm*40-50mm, giving you a stable shelf. I have done this on some long shelves for a bookshelf with large books that otherwise tend to bend.
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