Totally agree that this functionality needs to be added.

Workaround for now:
Upload a file to Proton Drive, create a link and paste it into the calendar entry. PDF and JPG tested and is opened in a browser upon click on the link, no login required.

Great idea and the closest existing I have identified so far for a better integration :

In a Calendar event, I would like to attach (or link to) a photo or a document (in the Drive / Docs), relevant to the event. I'd like to have some minimal formatting options in the description too. That will make for a better integration and a great time-saver.
Also, it would be awesome to have some sort of a visual index / identifier for the events ! Imagine a small logo, a thumbnail photo or document preview shown as part of a/the quick summary look-up when hovering over an event.

Adding my voice to this one. Trying to leave iCloud calendar, and I often add a picture or a PDF to calendar events on that solution. Typical use case 1: Kid comes home from school with paper birthday invitation. I add it to the calendar, and take a picture of the invitation that I add to the calendar event. Typical use case 2: I buy concert tickets months ahead of the concert, and receive the tickets as PDFs with a QR code in a mail. I add the concert to the calendar as an event, and attach the PDF so I'll have the ticket/QR code handy when I'm in queue for the concert.

Yep, looking for this too. If Apple Calendar and Outlook have this I feel like this needs to be a feature if users from those platforms are looking for something better than those options.

Attach tickets for a show, a trip, or anything else.

This is actually several different suggestions. Can we treat it as the one in the headline, i.e. "attach files to calendar event"? That's what I'm voting for.

Came here to ask for attachments to calendar events, as this is a feature in Outlook for instance, that I regularly make use of and really miss in Proton Cal.
Have to say, i fully support all 3 of OP's suggestions above though :)

I'd like to have a feature for Calendar that allows attaching a file to a calendar entry -- maybe at least for people with paid accounts -- so we are able to fully utilize features of all three -- Protonmail, Calendar, and Drive -- seamlessly integrated.

For my work, it is absolutely critical to be able to attach files to calendar events.
I regret that I will have to stay on Google Calendar until this is possible.
Also it should be possible for approved third parties to post invites to my calendar.

This feature is crucial to move away from my current iCloud calendars

Being able to attach files to Calendar events is pretty important and extremely useful when sharing files not necessarily suited for Cloud Drive storage. It would make Proton Calendar more competitive with Outlook in this respect. Thanks!!

For me attach file to an calendar event is a go no go to continu with Proton

One of the main blockers in my switch is the ability to add PDF attachments from Proton Drive to the calendar.

When I buy tickets to a show, I want them easily accessible on my calendar. Very important!! The file could upload to Drive then link, totally fine.

I would like to use Proton Calendar for school, but I can't because I can't upload documents (homework and syllabus PDF & DOC docs) to an event in the calendar. I'm still forced to use Google Calendar for this. Please add so I can get away from all tentacles of Google!

How about pics as icon? Like a photo of the person whose birthday it is that displays like an emoji in the title of the event? Like this:

🥷🏻 Ninja's Birthday

Vital that this feature is added - even if the capability (and this would be second best) is to a file that is stored in Drive.

Allow files to be inserted into appointments

I use Google calendar using its function of attaching files from Google Drive.

I scan sensitive documents, saving them in Google Drive, so that when the day of the event comes, if I forget the original, I have it on hand from any device.

A function like this in Proton Calendar, in coordination with Proton Drive and its encryption system, would make life easier for tons of users in its ecosystem.

Uso Google calendar utilizando su funcion de adjuntar archivos desde Google Drive.

Escaneo documentos sensibles guardándolos en Google Drive, para llegado el día del evento si se me olvida el original, lo tengo a mano desde cualquier dispositivo.

Una función así en protón calendar en coordinación con Protón Drive y su sistema de cifrado, facilitaría la vida a montones de usuarios de su ecosistema.