Deze simpele oplossing vond ik via google.
I've finally solved this problem once and for all - it certainly worked for me!
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 isnt what youre looking for, to restore inline attachment viewing type (again, without the inverted commas):
"defaults write com.apple.mail DisableInlineAttachmentViewing -bool false"
Restart Mail and youre back to normal.