I am using 3.0.1 with Joomla 3.9.1 and it doesn’t appear to be handling the extensions properly. It will display the file correctly (eg. File.pdf) but the link will have it duplicated (eg. File.pdf.pdf). This behavior is with show extensions on or off. This is with a fresh install as well as an upgrade from 2.9. Reverting to 2.9 works.
I have the same problem since upgrading to folder listing 3.0.1 and Joomla 3.9.1. with the see extensions turned on there is a ‘.pdf’ added to the file name. example – actual file name is CUSTDTL.PDF in the folder listing table it is showing CUSTDTL.PDF.PDF and when set as a link it downloads an html file ‘CUSTDTL.PDF.PDF.html’ not the pdf file.