I've twice written on the subject of inline attachments in Apple Mail. Those posts have been fairly popular so it seems I'm not the only one who has a dislike for them.
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 previously recommended two utilities to manage in-line attachment, Clive Galeni’s Anti Attachments Plugin and Attachment Tamer. Sadly, it appears that both are now defunct.
Worse yet, Clive Galeni seems to still be selling his plug-in, which I personally have found does not function properly with Sierra and have uninstalled. I've had several people write me and tell me they've also had trouble with the plug-in under Sierra or have recently bought the plug-in without reply from the developer. I too have reached out to the developer several times with no reply.
For now, The only way I know of to remove inline attachments are to use a terminal command. Here's how:
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).
You should know 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
As a side note, I've had a few people send me angry emails because they bought Clive Galeni’s Anti Attachments Plugin on my recommendation from January of 2016, a time when the plug-in was fully functional, in active development and worked with the then current OS. It's impractical for me to go back and update past posts when there is a change of circumstances. As always, I recommend you do your own due diligence, have good backups and take advantage of free trials.