Document plain text mime types in prompting tutorial

[pcoet]

Description of the change

Motivation

Type of change

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Checklist

• [ ] I have performed a self-review of my code.
• [ ] I have added detailed comments to my code where applicable.
• [ ] I have verified that my change does not break existing code.
• [ ] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [ ] I am familiar with the Google Style Guide for the language I have coded in.
• [ ] I have read through the Contributing Guide and signed the Contributor License Agreement.

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