FEATURE: Added max file size by type for uploads

Adds the ability to specify the max file size for a type via YAML config.

```yaml
Upload_Validator:
  defaultMaxFileSize:
    '[image]': '1m'
    '[doc]': '5m'
    'jpeg': 2000
```

These settings can still be updated on the instance variable via the method `setAllowedMaxFileSize`.

You can specify an App Category to limit the max file for rather than each individual file extension. This uses the built-in `app_categories` list. To specify an App Category in the config, just use the square brackets `[` and `]` around the name. This is to differentiate between an extension being limited and an entire category being limited.

With this update, it also adds support for `File::ini2bytes` when setting the allowed max file size.

Yep, you're right. Just did a test locally and null-values passed to `strtolower` return an empty string and thus pass the `isset` check. I'll make the change soon.