Illustration

1. Start JabRef
2. Open test-library-with-attached-files.bib contained in src\test\resources\org\jabref\logic\search.
3. Select entry minimal-sentence-case
4. Change citation key to "x"
5. Navigate to tab "General"
6. Drag and drop "minimal-note-all-upper-case.pdf" to the "File" box
7. See that nothing happens

Expected behavior:

• File already linked: There should be a notification (dialogservice.notify(...)) that the file is already linked in the library and thus is not relinked
• File not already linked: Link the file

Note: This is kind of detective work

Drag and drop onto the entry in the main table works.