JabRef 4.3.1
Linux 5.0.9-arch1-1-arch amd64 
Java 1.8.0_212

• I have tested the latest development version from http://builds.jabref.org/master/ and the problem persists

For some reasons, the document download (File -> Get Fulltext) would sometimes name the downloaded PDF without the filetype. So that the file is named: Smith2016 - TITLE and not Smith2016 - Title.pdf

I can not reproduce why is that so. The last DOI for which this happened to me: https://doi.org/10.1080/09537325.2013.850158