ICS Already Include in Body for Outlook/Gmail... but not for Office 365

[RulianZB]

Hi, i got a big problems with the mail including ICS.
I am using Laravel 11 and spatie/icalendar-generator 2.8

First i was including the ICS like an attachment :

attachData($this->generateLeaveICS($this->data['leave']), 'name.ics', [
    'mime' => 'text/calendar; charset=UTF-8; method=REQUEST',
])

But our security blocks ICS files, so I had to include these ICS directly in the body of the Mail via a symphony function.

$mailable->withSymfonyMessage(function (Email $email) use ($leave, $mailable) {
     $ics = $this->generateLeaveICS($leave);

     $htmlContent = (string) $mailable->markdown('mailing::email')->render();

     $htmlPart = new TextPart($htmlContent, 'utf-8', 'html');
     $calendarPart = new TextPart($ics, 'utf-8', 'calendar');

     $email->setBody(new MixedPart($htmlPart, $calendarPart));
});

Since I put this code in place, the invitation is interpreted correctly by Outlook and Gmail, but keeps an ICS file named ATT00001.ics on outlook.office365.com. However, I added specific headers to MSO such as X-MS-Has-Attach, X-MS-TNEF-Correlator, Accept-Language, Content-Language, x-ms-exchange-calendar-series-instance- id. I even tried to transform multipart/mixed into multipart/alternative but without any success. Do you have any ideas that could help me move forward on this issue?

Here is my ICS content that is validated by iCalendar Validator :

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Microsoft Corporation//Outlook 16.0 MIMEDIR//EN
NAME:CONGES
X-WR-CALNAME:CONGES
METHOD:REQUEST
BEGIN:VEVENT
UID:670e87ac9b0fe
DTSTAMP:20241015T151804Z
SUMMARY:CONGES
ATTENDEE;CN=Admin:MAILTO:[email protected]
DTSTART:20241017T000000
DTEND:20241017T235900
END:VEVENT
END:VCALENDAR