August 30, 2012
Some research the other day led me to this helpful blog post from 2009. While it does not solve the issue of attachments in Mail being displayed inline, it does stop the attachments from showing a preview of a PDF and/or image (you can also do this by right clicking on the image and can select to view it as an icon/normal attachment).
However, if you would like to make this the default behavior you’ll need to use the Terminal to set the preference. Terminal is in Applications>Utilities. Open Terminal and type:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
That will make every attachment you send act like an attachment instead of a pretty unusable decoration.
If you decide this isn’t what you’re looking for, to restore inline attachment viewing type:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool false
Restart Mail and you’re back to normal.