php is altering original filename

PHP's $_FILES['send_file']['name'] does not equal Content-Disposition's filename="..." value.

PHP is doing some sanitizing: removes path and stores only name of the file, like it said in documentation:

$_FILES['userfile']['name']

The original name of the file on the client machine.

Removing path means that everything before last slash/backslash (including them) is removed, that's what you got.