fix: asset entries in the manifest do not preserve the original file extension

[ElMassimo]

Description

This pull request is a draft to fix #8764, it currently modifies the backend-integration test so that it detects the bug described in #8764.

What is the purpose of this pull request?

• Bug fix

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
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• Ideally, include relevant tests that fail without this PR but pass with it.

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[ElMassimo]

Closing, as I intended this to be a draft PR instead.