An issue was discovered in GNOME Evolution before 3.35.91. By using the proprietary (non-RFC6068) "mailto?attach=..." parameter, a website (or other source of mailto links) can make Evolution attach local files or directories to a composed email message without showing a warning to the user, as demonstrated by an attach=. value.
https://www.nds.ruhr-uni-bochum.de/media/nds/veroeffentlichungen/2020/08/15/mailto-paper.pdf