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