For some reason, My MediaWiki install isn't creating thumbnails for jpegs. I've tried uploading 2 jpeg images and the File: pages show a blank placeholder image, reporting 0 x 0 as dimensions, and having an _error -1 metadata while able to show the file size. The files got uploaded correctly as they're there in images/x/xx/. Both images aren't big, they're around 1280x720.
I've tried uploading a png image, however it does work fine.
I then tried debug logging while reloading the File: pages, hoping to see some ImageMagick command failures as I'm using ImageMagick 7 and there's no mention of using "magick.exe" instead of "convert.exe" in any MediaWiki manual so far, however it turns out that I can't find any trace of thumbnails in the File: jpeg pages (tried searching for "thumbnail", "TransformationalImageHandler", etc. only found in the png page). No empty folders in thumbs either, it looks just like the whole thumbnailing part got skipped.
So I guess my question is how do I turn on/fix JPEG thumbnailing?
PS. tried uploading a small 10 x 10 jpeg, thumbnail still fails so it's definitely not some filesize/dimension limiter