bat

One BAT file of 64 bytes. A batch file is a Windows command script, and if a user double-clicks one that came from a download, it runs. That is why upload filters and email gateways block it. This one only echoes a harmless line. Use it to check that your site rejects .bat, that the check also works for uppercase .BAT, and that the message tells the user which types are not allowed. Also try uploading it inside a zip, to see whether your server looks inside archives or only at the outer file.