fix: detect octet-stream ICS attachments

[tayfuryldz]

Fixes #13688

Summary

• treat application/octet-stream attachments as calendar events only when their filename ends in .ics
• keep existing text/calendar and application/ics behavior unchanged
• add focused coverage for uppercase .ICS and negative MIME/extension cases

Validation

• git diff --check
• PHP is not installed in this local environment, so PHPUnit and php -l could not be run locally; CI is left to perform PHP validation

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[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Updates calendar attachment detection for .ics files labeled application/octet-stream, preserving existing MIME behavior.

Changes:

• Added case-insensitive .ics filename detection.
• Added positive and negative unit-test coverage.

File	Description
tests/​Unit/​Controller/​MessagesControllerTest.php	Covers MIME and extension combinations.
lib/​Controller/​MessagesController.php	Detects octet-stream ICS attachments.

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[kesselb]

Thanks a lot for your pr 👍

Looks good 🙏

Please sign off your commits: https://github.com/nextcloud/mail/pull/13732/checks?check_run_id=108473040545

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[tayfuryldz]

Updated the commit trailer to the AGENTS.md format and kept the DCO Signed-off-by trailer. The branch now points to f4374d8.

[kesselb]

/backport to stable5.12 please