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

• [x] 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).

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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