explanation from mac support forums : If the document is just a single page; Mail in Mac OS will show you the actual document rather than an icon for the attachment. Other mail applications do not do have this feature so they’ll just show an icon for an attached document regardless of how many pages the attachment has.
turn off :
defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
trun on :
defaults write com.apple.mail DisableInlineAttachmentViewing -bool false