Show attachments as icon by default in Mail OS X El Capitan
I just pasted the following into terminal and it works correctly for me. All attachments appear immediately as icons in El Capitan Mail:

defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes


It appears to be exactly the same as you entered it above. Before entering the command, I closed Mail and relaunched it after the command had been accepted in terminal. Don't know what to say other than it worked for me. Hope this helps someone!