Scream Tracker 1 & 2 module
From Just Solve the File Format Problem
.stm (Scream Tracker Module) files are modules created by versions of Scream Tracker prior to v3.0, where the format switched to the better known Scream Tracker 3 module.
It appears there may be more than one version of the STM format (or possibly just some confusion surrounding it). The format specification given below states "The ScreamTracker 1.0 format was the module format used by the ScreamTracker before version 2.0". Wikipedia notes that "Versions prior to 3.0 created STM (Scream Tracker Module) files" and MultimediaWiki lists STM solely as the Scream Tracker 2 module format (the Modland FTP also categorises .stm files solely as Scream Tracker 2 modules).