Allow for a setting so you can stop unknown file types from being imported

Current behavior: Any file type can be imported, if it's unknown it is converted to .txt

With this setting: By default normal behavior is persistent however adding setting (as per template file) stops these files from being imported and shows error to user.

Should be pretty obvious what it does and how it works.  Don't think we need docs as the docs are in the template file.

It would be nice in the future to allow for an array of supported file types to be passed but that's a job for the future once this is in and mature.