Mail attachments
One of the longest-standing complaints I see about Apple’s Mail app is that it places any images you drag into an email as inline images (which means the image appears in the email wherever you actually drag & dropped it) instead of as a standard attachment (which appears as an icon at the bottom of the email regardless of where you drag & drop it). The problem is that inline images (and PDFs) don’t always work well with all email apps on the receiving end. Some people complain that the image quality gets degredated, and in some cases the image doesn’t come through at all.

Apple doesn’t offer a way to change the feature easily. You can choose the “Send Windows Friendly Attachment” option when attaching the file, and even right-click on the image and choose “View as Icon” – and it’s still iffy if it’ll work.

Cheap Solution (works better)

Clive Galeni has the only GUI solution I could find called AntiInline. It’s a plugin for Apple’s Mail app and it solves the problem perfectly. Once turned on in the Mail preferences, all files placed in an email (regardless of how you do it) show up as true attachments at the bottom of the email.

AntiInline also has the option of allowing to keep images in your email signature remain as inline images so they show up the way you would expect. You just have to include “email-signature” (without quotes) in the filename of your signature image.

AntiInline is offered in a version for macOS ElCapitan, Sierra and High Sierra and costs $14.90 and runs on up to three Macs per license.

Free Solution (works, but works too well)

There is a FREE solution, and it’s simple if you’re familiar with using the Terminal app. Fire-up the terminal and enter the following:
defaults write DisableInlineAttachmentViewing -bool yes
Just know that this turns off inline images entirely, including any images you may have in your email signature. To revert back to the default inline mode, simply replace the “yes” at the end with the word “false” (without quotes).

I’m not a fan of images in email signatures, so I’ve decided to just turn off images completely using the Terminal code method.