Mine are all from attachment thumbnails, via an add-on.
reverted to "on" because I haven't noticed any improvements. I did, however, change sendfile_max_chunk from 512k to 1024k.... *shrug*
here's a line from the error log: Code: 2020/05/08 00:31:40 [alert] 13630#13630: *33911 pread() read only 8929 of 10563 from "/home/nginx/domains/domain.com/public/data/featured_images/featuredimages_thumb_4.jpg" while sending response to client, client: x.x.x.x, server: domain.com, request: "GET /data/featured_images/featuredimages_thumb_4.jpg?1588897900 HTTP/2.0", host: "domain.com", referrer: "https://domain.com/" EDIT: they do all seem to be .jpg.
So, to be clear, they start as jpgs - they show up as broken images on the site... not sure what's being appended there in the log entry.