When I try to create <Options><Custom Entry Types> it provides only the BibTeX fields in the dropdown list. I have BibLaTeX mode selected in <Options><Preferences><Advanced> in Jabref 2.9.2. You can type the field name in, so it is a convenience and logic fix, as BibLaTeX takes over. Maybe a tick box option in the pop-up panel if some want to retain backward compatibility.
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