It is the actual file. The filenames that are temporary on the server are ofcourse randomized. You can check that there was no error using $_FILE['attachment1']['error']. If its higher than 0, there has been some error in the upload. Check the manual for the error codes.
BTW, you dont have to use quotes around variables(and you shouldnt).
So this style is better:
$mail->AddAttachment($file1_path, $file1_name, "base64",$file1_type);
Ofcourse you CAN use but usually theres something else in there too. And if you use variables inside quotes, they should be put inside brackets. For example:
$name = 'cahva';
echo "Hi, my name is {$name}.";