I have a bunch of tags set up with extensions listed, but the only thing I can do with them is manually tag files as they're downloading. I swear FDM used to read the file extension from the box and tag/sort them automatically but it doesn't do that any more, even after reinstalling. Is there a fix or is it just a bug?

Hello,

Works fine for me. You need to provide detailed steps on how to reproduce the issue if you want to get help.