We noticed a bug in udig were certain supported file types could not be drag into udig. An example of this is dted while dt2 would load dt0 and dt1 would not. We tracked this down to the plugin 'net.refractions.udig.catalog.ui''s file 'net.refractions.udig.catalog.ui.FileConnectionFactory'. The method public URL createConnectionURL( Object context ) needs to be changed to the following.
I have also attached the file.