test: cover filename extension helpers

[sgallese]

Summary

• Add unit coverage for filename extension edge cases.
• Cover multi-extension, hidden-file, and trailing-dot behavior.

Testing

• ./gradlew --no-daemon testGplayDebugUnitTest --tests com.nextcloud.android.utils.ExtensionsTest --rerun-tasks
• ./gradlew --no-daemon spotlessKotlinCheck

Screenshots

Not applicable, test-only change.

Checklist

• Tests written, or not not needed

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[alperozturk96]

@tobiasKaminsky Locally tested works but Unit Test is not running, fyi.

[sgallese]

Looks like Codecov upload step is failing on CI (gpg: Can't check signature: No public key) but the unit tests are passing.

Seeing this error on a few other PRs.