1) Create a new thread (test)
2) In the quick reply, click 'Upload a File' and select a few images
3) Once upload is finished, hit the 'Thumbnail' button to insert all images and click 'Post Reply'
4) Try uploading more images by repeating step 2, the upload stops and here's what I see:
The upload process is not visible after selecting images for the 2nd time, but saving the draft and hitting the reload button shows that they are processed server side.