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.