The file is attached in mail compose window Add attachments to emails in Mail on Mac - Apple Support
Add an email attachment in Mail on iCloud.com - Apple Support
Mail Drop limits - Apple Support
With Mail Drop, you can send attachments up to 5 GB in size. You can send these attachments right from Mail on your Mac, the Mail app on your iPhone, iPad, or iPod touch, and from iCloud.com on your Mac or PC. All files types are supported and attachments don’t count against your iCloud storage. If a message, including its attachments, is larger than your Internet Service Provider (ISP) limit, Mail will ask you to send the attachments using Mail Drop.