dragging a binary to Leo should create @url, not @edit

This is rather a feature enhancement request, then a bug, though it can lead to Leo stalling.
Dragging files that are not normally edited directly, like PDF files, should create an @url link to them, rather than an @edit or @auto node. What kind of node is being generated by which files should be settable via @setting entries.
The default link created for local files should be *relative* rather than absolute, in my opinion (even if that includes going a couple of levels towards the file-system root). The number of levels before an absolute path is created may also be a setting.