As everyone certainly noticed since yesterday the Opengeofiction site is labeled as insecure. I hope the issue will be resolved in reasonable period of time however now it makes impossible to upload and download files with the most typical version of JOSM (.jar files, used also in Windows JOSM installator).
Using hit and miss method I found simple solution to that problem. Go here: https://josm.openstreetmap.de/ and download the .jnlp version of JOSM. It is slightly more annoying to use (starts longer, for Apple computers users it may appear a bit blurry) but at least remembers all settings and plugins from .jar version. After you try to upload and download using .jnlp version you will receive the window with the content pretty similar to the one from web browsers where you can allow JOSM to download and upload from "insecure" website called OGF.