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.