I've checked in a quick tweak in r79029 that adds thumbnail previews of image files on Special:Upload before you submit the form, in browsers supporting the HTML 5 FileAPI [1] (such as the current stable releases of Firefox and Chrome).
This helps with a longstanding problem that's bugged me for years: when the AJAX filename conflict thingy trips, you get a thumb of the *existing* file but not of the file you're about to upload, making comparisons harder! Now you've got the file you just selected sitting right above it, which is rather more convenient.
Files larger than 10MB aren't previewed as a guard against accidentally hitting a really big file that eats your browser's memory and kills everything. :)
On browsers that don't include the FileAPI FileReader class, or when selecting a file that isn't a PNG, JPEG, GIF, or SVG file, you won't see any change in behavior.
[1] http://www.w3.org/TR/FileAPI/ [2] https://developer.mozilla.org/en/using_files_from_web_applications
-- brion vibber (brion @ pobox.com)