출처 : https://discussions.apple.com/thread/4026330?tstart=0

defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes

Open: Applications>Utilities>Terminal and type (without the inverted commas):

"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 (again, without the inverted commas):

"defaults write com.apple.mail DisableInlineAttachmentViewing -bool false"

Restart Mail and you’re back to normal.