Filename extensions (. something at the end of a filename) tell programs and users what type of data a file contains. In QNX filesystems, extensions are just an ordinary part of the filename and can be any length, as long as the total filename size stays within the 505-byte filename length limit.
Most of the time, file extensions are simply naming conventions, but some utilities base their behavior on the extension. See Filename extensions for a list of some of the common extensions used in a QNX Neutrino system.