I was just playing around with the latest build of Blender 2.8 and downloaded some files from the internet to check out Eevee with. I wanted to open the file from my downloads folder and noticed it is not a default bookmark, although there is a 'System Bookmarks' folder in the file browser, and most OSes have a system download folder.
It doesn't seem to be possible to add bookmarks to 'System Bookmarks', so I added my download folder to the 'Bookmarks' folder instead. I know there is a bookmarks.txt file in the Blender config folder, but it doesn't contain the System Bookmarks category and trying to add one to it doesn't work. Is it possible to modify system bookmarks somehow?