Your comments

My proposals:
- Make the content- and link-fields of the note editing dialog a drop target for files; dropping a file will insert a link at the current cursor position.
- Allow three modes:
+ [drop]: merely a link
+ [Shift/Ctrl+drop]: move/copy the source file to a CN-subfolder, make the target's name unique by combining the original filename with the note-id
- The displayed link ("file://..." or whatever) should reflect the "mode" - external or CN-internal
- Give an alternative way to insert link by adding a browse button (with keyboard shortcut) according to the former comment.

The subfolder should be configurable, so, e. g. from dropbox-view, I can decide whether to upload the "attachments" or not. For the same (and many more other) reason(s) the files should not be included in the database. (Or should I say "must not"?)