Cannot validate types if DataExtensions are used

[elliot-sawyer]

The validate method on Link provides a hook for extensions; however, it is hard-coded to validate on $this->Type rather than $this->owner->Type.

Steps to reproduce:

1. Replace TreeDropdownField with UploadField via an extension hook: $fields->replaceField('FileID, UploadField::create('FileID'));
2. Try to choose an existing file and attempt to save
3. Observe validation error: Please select a {LinkType}