I've been going kind of crazy covering a particular Drupal site I'm building in Behat tests—testing every bit of core functionality on the site. In this particular case, a feature I'm testing allows users to upload arbitrary files to an SFTP server, then Drupal shows those filenames in a streamlined UI.
I needed to be able to test the user action of "I'm a user, I upload a file to this directory, then I see the file listed in a certain place on the site."
These files are not managed by Drupal (e.g. they're not file field uploads), but if they were, I'd invest some time in resolving this issue in the drupalextension project: "When I attach the file" and Drupal temporary files.
Since they are just random files dropped on the filesystem, I needed to: