Fix calendar emails to be outlook compatible

[ChristophWurst]

• Resolves: Calendar Invites - Outlook integration #12885

Summary

Resubmit and formatted version of #34324

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[ChristophWurst]

@miaulalala would this be realistic to finish for 26?

[miaulalala]

Questionable if symfony supports this: symfony/symfony#44557

I will look into it tomorrow

[miaulalala]

The attachment is embedded now, but I have no outlook client to test this. @merlinwoff can you please try?

[ChristophWurst]

The rebase went sideways

[ChristophWurst]

🥲

[nickvergessen]

Please mention in the 27 changes issue

[nickvergessen]

27

[nickvergessen]

?string $contentType = null

[DaphneMuller]

Hello!
Thank you for your work on this! I noticed that your PR still has the label 'documentation'. I'm not sure if you already had a look at the documentation for your PR, but at Nextcloud we strive to document changes that affect other app developers or other admins before the release takes place, and I'm pinging you with the hope to get this done.

Are you familiar with our documentation process already?
Changes that affect app developers should be documented here:
https://docs.nextcloud.com/server/27/developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_27.html

Changes that affect administrators should be documented here:
https://docs.nextcloud.com/server/latest/admin_manual/release_notes/upgrade_to_27.html

While I understand sometimes it's handy to merge fast to get your changes in, it would be great if next time you could try to add the documentation before merging. You can find more information on that here:
https://docs.nextcloud.com/server/27/developer_manual/prologue/compatibility_app_ecosystem.html#documentation-procedures-of-changes-that-affect-app-developers

If all your documentation efforts are done, please remove the label 'documentation' and check the checkbox in your opening note and I'll stop bugging you :)

Many thanks in advance and thanks again for your work!