Apple Mail: send attachment

I'm attaching a PDF to an email. It is appearing as an open document in the body of the email. I want it to be as an attachment, not as the PDF itself in the body. I found a solution that says to enter the following in Terminal:

defaults write com.apple.mail DisableInlineAttachmentViewing -bool YES

I closed the Mail application, entered the line above in Terminal, and opened Mail. It didn't work. I did find that you can right click the inline attachment in each email and select "View as Icon", but I was hoping to be able to do something that would affect all emails.

By the way, at the top of the Terminal window, this appears:

The default interactive shell is now zsh. To update your account to use zsh, please run `chsh -s /bin/zsh`.

I did not do this. Is this why Terminal didn't work?