Undefined file type in JabRef 4.2

I have installed JabRef 4.2 on Windows 10. When I select the “Synchronize file links” from the drop-down list of “Quality”, I always get this warning message.
How should I do ?
Does anybody familiar about this situation ?
Thanks very much.

I can confirm the same error under MacOS 10.13.4.

As far as I remember synchronize file links is deprecated and you should probably use “Automatically set File links”.

The other option would be to check Options-> Manage External file types and check if you have PDF defined.

Ok, Thanks for your information.

Ok, I will try it. Thank you very much.

The pdf type is defined already, but it still shows that ‘PDF’ is undefined.

While, if I used the Jabref version 4.0, it will be ok, but it still shows PDF undefined in version 4.3.1