Unlike Entourage and Outlook, Apple Mail follows its own guidelines for whether an attachment should display inline (as in, you see the entire attachment right in the message) or as an icon (as in, you have to double-click the attachment to view it). To toggle an attachment’s display method, right-click the attachment within your unsent message and choose View As Icon or View In Place. To force attachments to always display as icons, use this Terminal command:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
Used without permission, so here's the link back: http://www.maclife.com/article/howtos/10_ways_soup_apple_mail?page=0,1.