In an effort to maintain Chromium's security model OpenFin removes Node from the renderer process (which is enabled by default in Electron). Access to the file system in OpenFin is restricted exactly as it is on the web. There are options such as FileSystem API and File drag and drop that allow for limited interaction with the user's permission.
Group Policy allows for some more control. e.g. the Chromium policy settings PromptForDownloadLocation allows directly downloading a file without prompting the user. Here is the complete list of Chromium Policy Settings.
NOTE: some settings can only be set through Group Policy and cannot be set directly in the registry.