Orcs and prisoners are indeed Shelob's diet:
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And Orcs, they were useful slaves, but he had them in plenty. If now and then Shelob caught them to stay her appetite, she was welcome: he could spare them. And sometimes as a man may cast a dainty to his cat (his cat he calls her, but she owns him not) Sauron would send her prisoners that he had no better uses for
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–"Shelobs Lair",
The Two Towers.
I can't remember if the film mentioned what Shelob usually ate.