Are email Attachments technically always inline, or does macOS Mail not allow conventional, non-inline attachments?

In macOS mail, you are forced to attach files inline in the message. I am aware you can enable "Always Insert Attachments at End of Message" in the menubar's Edit menu, but I understand this still leaves the attachment inline. Even when an image or PDF is shown as an icon at the end of the message in macOS Mail, the recipient ends up seeing the attachment as a huge inline image.

Is there an actual difference in "inline" vs. "non-inline" attachments in email protocol? Or do other apps technically attach things inline at the end, just visually showing it differently?

Solution 1:

There is a definition in the MIME specification for both inline and attachment files within email messages, but (from the Wiki page linked above):

As of 2010, a good majority of mail user agents do not follow this prescription fully. Most users are unaware of how to set the content-disposition to attachment. Many mail user agents also send messages with the file name in the name parameter of the content-type header instead of the filename parameter of the content-disposition header. This practice is discouraged.

In short, whether the sender and recipient follow the spec, or do things differently, is uncertain.