Possibly related to this admin option:
Hello users NodeBB, how can you block the ability to download files from the forum for guests?
Go to /admin/settings/uploads and turn on
Make uploaded files private
@baris Can we enable private files only for files (.pdf.txt, etc) but leave viewing images for everyone? Because currently if you enable this feature, the images are also private for guests which is not ideal.
Not right now, feel free to open an issue for it on github page.