Did you know that Apple Mail does the presumptuous thing of sending all images (and maybe other attachment types) as inline attachments, no matter what options you choose in the program, like attaching at the end of message, like making it “Windows-friendly” (as if it’s a Windows problem),…. Nor does any of the very typically low-quality “solutions” you’ll find by misunderstanding Apple users online help, like changing message type to plain text or running that one silly line on the command line

defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes

Just look at that and think about what it does! None of those do anything at all.

But I found this today after much searching: OS X Mail – disable inline attachments, and it is indeed the only solution that works.

Edit: However, I can no longer recommend this Mail plug-in. Not only does it nag-screen for payment an hour or so after first using it, there is not even a way to uninstall it without manually going through the system. Given how difficult it is to cleanly remove .pkg installations on OS X, I consider this thing to be in terrible taste. Use at your own risk.