Fail blacklisted file uploads immediately and notify user

I want users to get an immediate error/warning when they try to upload a file with a blacklisted extension (like .eml or .zip) in comments. Right now, the file appears to upload successfully and displays a download icon, but because it is blocked for security reasons, it is never stored on the servers and cannot be downloaded. It is important to notify the user immediately during the upload process and explain why the file is rejected, so they don't think their upload succeeded when it actually failed.