Missing [file] on line 355 in attachments controller

Hello,

Not sure if this is intended. Was having issues uploading in media endpoint and found that the code on line 355 was looking like this:

```
$file = wp_handle_upload( $files, $overrides );
```

Changed it to:

```
$file = wp_handle_upload( $files['file'], $overrides );
```

Which fixed uploading issues for me(using form-data).
