UPDATE: February, 2017: It appears Clive Galeni’s Anti Attachments Plugin is no longer being supported and does not function properly under macOS Sierra. I no longer recommend this method.
By default, Apple Mail sends certain e-mail attachments such as images and PDFs inline, or in the body of the message. While this may look visually appealing for some messages with images, it can be frustrating and confusing. I personally don’t like Inline attachments, so here are two ways to get rid of them:
If Mail is open, quit it. Open the Terminal (Applications → Utilities) Enter the following command:
defaults write com.apple.mail DisableInlineAttachmentViewing -bool yes
Once you restart Mail.app any email messages that you send or receive will display attachments as icons, not inline. If you later you decide you want to change your preference back to viewing attachments inline simply repeat the above command but change the “yes” at the end to “false” (without the quotes). Note that this method is universal. Meaning all attachments, including those that are simply graphics (such as in email signatures) will be sent as attachments - so your messages may no longer have the desired appearance.
Clive Galeni’s Anti Attachments Plugin
The terminal command is free and either turn inline attachments on or off. Simple enough. But my favorite way to handle attachments is by using Clive Galeni’s Anti Inline Plugin. This third party add-on gives you additional control over attachments handing including the ability to leave signatures inlined thus preserving the appearance of mail images. The cost is $14.90 but I don’t mind paying for a utility I use every day.
I previously recommended another plug-in called Attachment Tamer but unfortunately that developer hasn’t been able to keep up with OS X development so I can no longer recommend it.