Attach images doesn't work

Each time I try to put a JPG or PNG file in a post, by clicking the “upload” button and choosing the file OR by pasting it directly into the edit box, I get an error message “Sorry, there was an error uploading that file. Please try again.”

Please try a different Internet Browser such as Chrome.


Indeed, Chromium works for image uploads. I can even upload the image with Chromium, switch back to Firefox, and the image is in the draft in Firefox.