Add MediaType property to DbContextFactory

[CedricDumont]

Signature for method in class DbContextFactory are like:

    public static T Create(string xmlfileName, ConnectionBehaviour connectionBehaviour = ConnectionBehaviour.Transient, string connectionId = null)

but Effort.Next supports xmlDataloader, CsvDataLoader and could be enhanced.

Better to add a property to tell the content type of the file application/xml, text/csv ....