Why does drag-and-drop generate this icon instead of the image?

jimprovising

While preparing to facilitate a session I've noticed that when I drag-and-drop an image into a board a "postcard" icon appears (attached). It says it's scanning for viruses when I try to open it, and when it does open, the new tab is blank/white/empty.

Yes, I've restarted, quit the browser, refreshed the page, and started a new Mural. Nothing helps.

😣