Hello!
Recently had a customer that sent over an attachment that was a PDF file and seemed to have been inlined in their email. Originally I thought they did not attach anything at all but it turns out the PDF was inlined, and I was only able to see it by downloading the source file of the email and opening it in another email program.
From what I can tell this shouldn't even be normally possible to do (Gmail for example doesn't even let me select a PDF file to inline) but maybe some specific email clients bypass this ability. Either way it's something that I wanted to mention for a potential fix or something that can be used to help our team out.