docs: add missing mime_type in digital products recipe

[shahednasser]

Fixes #5554

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Latest commit: 0154887

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Name	Status	Preview	Comments	Updated (UTC)
api-reference	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 6, 2023 0:24am
docs-ui	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 6, 2023 0:24am
medusa-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 6, 2023 0:24am