the cause of this issue is multifaceted like most problems in life.
It certainly is.
When there is html code present and you "drag-and-drop" your attachment into the email, it MAY get embedded in the middle of a bunch of html code, and Outlook and a few really stupid email programs will be unable to notice that there is an attachment in there.
That is why these sometimes improve things:
• [√] Send Windows-friendly attachments
• attach at very beginning or very end
• deliberately using the paperclip "attach" Icon rather than drag-and-drop
• send ONLY the attachment all by itself (nt)
** There is one more extreme option. Tell Mail that this message must NOT contain any html:
Mail > Preferences > Composing > Composing: Message Format: { Rich text (html) | Plain text (NO html) }
Only letters and numbers and special characters, and no html, will go out with your message. Your attachment now has "nowhere to hide".