When you attach certain file types (i.e. PDF, PNG, Jpeg) to an email in macOS Mail, a thumbnail of that attachment will be displayed (‘appear inline’) in the body of the message.
If you are only attaching a single file, the issue is not apparent. However, when you need to attach multiple files and they ‘appear inline’ the formatting of the email can start to look confused and the message requires scrolling through.
You can change the way macOS Mail displays file attached to your email messages.
defaults write com.apple.mail DisableInlineAttachmentViewing -bool YES