Below where it says "ATTACHMENT\nChoose files to attach to the post here. ...", I choose to upload a "png" file smaller than 1 MB as specified in the limits and then it displays a "The file could not be uploaded".
This happens in Safari, Firefox, and Chrome. Also if the file is called "test.png" (no spaces in the name or anything)
This mechanism used to work in the past, as seen here: http://www.juce.com/forum/topic/svg-scale/viewbox-problems