Ok, that sounds like it was completed blocked and doesn't even exist in PHP tmp to validate against. Most likely your servers mod_security is false positive flagging it. Try something simple like a text file that just says "hello" in it and see if that gets blocked.

It's being blocked or something is wrong with php tmp. Check with your host first to see if mod_security is blocking the uploads. If not you'll need to check tmp folder permissions.

That's a tmp directory problem. tmp doesn't exist or can't be accessed. So it's impossible for the PHP upload to take place. Check php.ini and ensure upload_tmp_dir is correct. The php.ini upload_tmp_dir should point to your servers root tmp folder. I think Joomla may override this with its tmp folder parameter in Joomla global configuration so be sure that's also correct and points to the /tmp folder in Joomla root. Check permissions on both if they both exist and are correct.