add plain text to multimodal prompting guide

[pcoet]

Description of the change

Update multimodal prompting guide to include plain text support.

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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[markmcd]

Can we make this wording sound less strict?

they are subject to the following limitations and requirements

It's not a strict requirement that plain text inputs adhere to one of these formats, since they don't map to an encoding like other media.

We should also recommend that users specify the MIME type manually if it isn't in this list. In this cookbook recipe I gave an example with a C++ file because that's not detected as plain-text, but it really is, so users can specify it.

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[pcoet]

Closing this and opening a new PR against the io-features branch: #401