All image attachments (jpg, png, pdf) are shown inline with the email text and not as icons like zip files. Sometimes there are too many attachments and it's harder to find the text.
I emailed myself a screenshot. The text is easy easy to overlook:
To hide an attachment image do a right-click on the image and select "View as icon":
The result looks like this:
If you want to undo the change do a right-mouse click on the icon and select "View in Place":
Can you permanently disable inline attachments? Not anymore.
In former - simpler - times it was possible to permanently disable showing the attachments inline. Not even in High Sierra I was able to get this working, let alone Mojave or Catalina. There is an old Terminal command:
defaults write com.apple.mailDisableInlineAttachmentViewing -bool yes
But it doesn't change the Sandboxed preferences. And when I change the Sandbox plist file from Mail with Prefs Editor AppleMail still shows the inline attachment.