File Block: Replace on-mount downloadButtonText effect with a default variation

[Mamaduka]

What?

PR replaced the on-mount useEffect that imperatively set the localized downloadButtonText default ("Download") with a declarative isDefault block variation, mirroring the Search block pattern.

This also fixed an odd bug with the File block in StrickMode, when reloading a newly saved post, it becomes dirty again. Related #73378.

P.S. I've wanted to remove this effect for a while. Props @t-hamano for suggesting this method in a different PR.

Testing Instructions

1. Open a post or page.
2. Insert a File block and select a file.
3. Confirm that the download button default text is correctly set.

Testing Instructions for Keyboard

Same.

Screencast

CleanShot.2026-06-16.at.17.25.05.mp4

Use of AI Tools

Assisted by Claude.

[github-actions]

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Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: t-hamano <[email protected]>

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[github-actions]

Size Change: +46 B (0%)

Total Size: 8.59 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-library/index.min.js	324 kB	+46 B (+0.01%)

_{compressed-size-action}

[t-hamano]

Nice enhancement 👍

[github-actions]

Flaky tests detected in 97d1cf4.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/27619317820
📝 Reported issues:

• [Flaky Test] should update the HTML only for the latest navigation #78835 in /test/e2e/specs/interactivity/router-navigate.spec.ts