"The name consists of a constant prefix plus my user-selected suffix" would constitute enough information. I was thinking more of cases where the prefix /varies/, e.g. because it's used for content addressing, or to serial number the files, or whatever other scheme is in use.
Most code I've worked on that downloaded arbitrary files and gave them names based on their origin (and thus which could be vulnerable to this attack) prefixed the filenames with some varying code or number such as a node identifier. So a naive "assume constant prefix" wouldn't hurt us, but if you could guess our naming scheme (and if we used btrfs) there's definitely a window.