fixes custom body responses having a html extension

[Lexedia]

Description

This is a followup on #597 that sets an approximate extension when using a custom body response.

Type of Change

• 🛠️ Bug fix

Ready Checklist

• I've read the Contribution Guide.
• In case this PR changes one of the core packages, I've modified the respective CHANGELOG.md file using
  the semantic_changelog format.
• I updated/added relevant documentation (doc comments with ///).
• I added myself to the AUTHORS file (optional, if you want to).

[docs-page]

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docs.page/schultek/jaspr~654

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[github-actions]

Package Version Report

The following packages have been updated:
jaspr_test : 0.21.7 -> 0.22.0
jaspr : 0.21.7 -> 0.22.0
jaspr_builder : 0.21.7 -> 0.22.0
jaspr_cli : 0.21.7 -> 0.22.0
jaspr_serverpod : 0.5.8 -> 0.6.0
jaspr_router : 0.8.0 -> 0.8.1
jaspr_content : 0.4.4 -> 0.4.5
jaspr_flutter_embed : 0.4.9 -> 0.4.10
jaspr_riverpod : 0.4.1 -> 0.4.2

[schultek]

Can you elaborate why this is needed?

You can already define Routes with an extension, e.g. Route(path: 'some/file.json', ...).

[Lexedia]

Well, it was mostly for routes without an extension set, although forcing one can also work indeed.

Tbh, it was mostly my fried brain at 2 o'clock that decided to make this PR