Hey Mischa-
The mongrel upload progress has to be in_front => true to work. That is the only way for it to know the status of the upload. Mongrel keeps an internal progress updater going and tells the upload progress handler to increment the progress bar in the browser. But control doesn't come to your rails app until after the entire upload is done. AFAIK there is no easy way to do the validation you want to do before the prorgess bar is done and the file is already uploaded 
Cheers- -- Ezra Zygmuntowicz-- Lead Rails Evangelist -- ez@engineyard.com -- Engine Yard, Serious Rails Hosting -- (866) 518-YARD (9273)