New FuelPHP Upload class compatibility problems

I am currently working on a site that is due to go live in March so I am updating Fuel regularly.

Here is my original code that worked with Fuel 1.5/develop.

```php
<?php

$config = array(
    'ext_whitelist'  => array('csv'),
    'path'           => APPPATH.'data/',
    'prefix'         => date('Y-m-d_H-i-s_')
);

\Upload::process($config);

if (\Upload::is_valid()) {
    \Upload::save();
} else {
    throw new \Exception("Invalid file uploaded, please try uploading a .csv file.");
}

$files = \Upload::get_files();
$data = file_get_contents($files[0]['saved_to'].$files[0]['saved_as']);
```

Updating to 1.6/develop causes this error when uploading a file.

```
call_user_func() expects parameter 1 to be a valid callback, function '/tmp/phpvJjKU0' not found or invalid function name
fuel/vendor/fuelphp/upload/src/FuelPHP/Upload/File.php:447
```

Straight away I noticed that line was using `call_user_func()` incorrectly, but also, it shouldn't have been getting there. Please bear with me whilst I submit the pull request to fix that issue.

I imagine there will be further issues with the `fuelphp\upload` class, as it doesn't appear to be complete. But this pull request, and my other pull request, at least make it work with my existing code.

I know that my pull requests are going to break uploading via FTP, if that worked to begin with.