Using the “mailto” protocol is as far as you can go with Airtable to communicate with a desktop email client, and as you’ve discovered, that protocol doesn’t support attachments.
I would recommend sending an email to support@airtable.com to request that they add a feature to communicate with desktop email clients, similar to how FileMaker can do this.
The best you can do is trigger an automation with Make’s advanced automations for Airtable to put an email with an attachment into the user’s Drafts folder, but it won’t launch their email client for them. They would have to open up their drafts folder to find the email.
Hope this helps!
— ScottWorld, Expert Airtable Consultant