Fix #680: Fail if placeholder resolves to binary data, that would break yaml

[olivergondza]

Description

Fail fast if placeholder is to produce binary data. This provides better error message to diagnose misconfiguration.

Fixes: #680

Checklist

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Type of Change

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
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