In Keepass and LastPass I can add small files to my password entries, typical keystores for HTTPS with client auth. Please add such a feature to ProtonPass.

This would be very useful indeed. As a user of Bitwarden, this is a feature I would really want before making a switch

This is critical. Key files, scans of IDs, etc.

I was very surprised coming from LastPass that attachments are not currently possible.

This is what's preventing me from switching at the moment.
I currently use KeePass and for some item it is critical to be able to attach files dorectly in the password database.
Examples are backup recovery files (e.g. for Synology or borgbackup), recovery codes for 2FA (that can be text, images or pdf; images are best for security).

This sounds like a perfect integration with Proton Drive! Attachments should be able to be added to specific password AND note entries in Proton Pass. Adding an attachment should transparently upload the file to a location on Proton Drive, and allow the attachment to be accessible at a minimum, from the password/note entry in Proton Pass, and optionally by going to the actual stored location within Proton Drive.

Proton is uniquely positioned as one of the few (only?) companies with a password manager and secure cloud storage as their core product offering. Google, Apple, Microsoft don't count here as none of them prioritize their password managers, and certainly don't care to properly integrate them with their cloud storage.

Useful file attachments could also be screenshots or PDF.
Maybe this functionality could be integrated with Proton Drive using a predefined "special" Pass folder.