I use this app. Since JabRef is built on the bib file, I wanted something that also relies on the bib and this app fits that very well. I link the same folder (on the cloud) where my JabRef stores my PDFs and the app can find them. I will note that the app already works if there are more than 1 PDF linked in an entry.
The app does explicitly call out JabRef regarding using the file field to find the attachments.