There’s a setting in mail that lets you send windows friendly attachments, but it’s well hidden.

when you use the paperclip to choose a file to attach, there’s an options button at the bottom left of the window, click that and there’s a tick box to

“Send Windows-Friendly Attachments”

make sure that’s checked.

Otherwise I think the attachments get embedded in your email which makes it hard for windows users to ‘unattach’ them.