I see what you mean. Yeah, this is a huge limitation of HTTP uploads. It's really amazing that in 2008, uploading is the same as it was in 1998. Even if Silverstripe did handle the error correctly, you'd still have to wait for the file to upload before you get anything back. That's why I'm a big fan of Flash uploads, and I built the SWFUpload extension to bring uploading features closer to where they should be. Filtering out invalid file types and invalid file sizes, displaying upload progress, multiple file selection, etc...