Apple - Turn off automatic preview in apple mail

In Terminal, type:

defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes

If you want to turn it off again, use

defaults write com.apple.mail DisableInlineAttachmentViewing -bool no

You may need to restart Mail after running this to change the behavior.

There is an extension for Apple Mail called Attachment Tamer. It has all kinds of settings, but the most useful is the one that defaults attachments to the icon view.

http://lokiware.info/Attachment-Tamer

It is $15 but well worth it.

To expand on @user43521's answer, there are two ways to solve this problem:

First solution:

If you drag-and-drop files—even images—into Mail.app's compose window, they'll be sent as real attachments (Content-Disposition: attachment).

Second solution:

If you open a terminal and enter this command, it will always make attachments real attachments:

defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes

You only need to do this once. To revert to normal behavior:

defaults write com.apple.mail DisableInlineAttachmentViewing -bool no

The Viewing portion of this is very confusing; this affects how your outgoing mail is sent. If you receive mail with inline attachments, they'll be displayed as expected. In fact, it's even more confusing than that, because regardless of this setting, both inline and attached images that you receive will be displayed inline.