I tried uploading a file that is not allowed, after 10min of uploading I finally got the message...
File-extension not allowed!
Perhaps better to first check the extension before uploading.
This is already noted but is lowpriority atm. To check for that i would have to use javascript on the submitbutton, and parse the filename from there, and decide wether to submit or error you in the face before actually submitting the form. But since it's already planned to replace the uploadform with Ajax, it will have to wait.
With ajax you will also be able to view filetransfer as you upload, with some other small goodies
Until then; Read what you are about to do before doing it, sorry for the inconvinience