Expand to other entity types / datasources

Having thought about this a little bit I was thinking in the settings form, get a list of the enabled datasources and then generate the list of availbable bundle options from that.

When I poked around at it for 5 minutes I stumbled get getting the list of datasources before i gave up for another day

Here's a patch for a feature change that allows customizing the File datasource (file entity) MIME type. These will be grouped with nodes with the same label. And put in "Other".

This solves for a specific use case (File entities) but long-term would like a more general solution.

Updated approach to add a `file_default` setting. All Files might want to be grouped in "Documents", for example.