Supreme Commander uses a variety of different file formats.
Units use .sca (animation), .scm (model) and .dds (texture).
Both units and maps are actually folders containing a set of files and units can be accessed by unzipping units.scd - which is actually just a .zip file.
Blueprint files for units and structures have the .bp file extension.
Scripts describing in-game logic are written in the Lua programming language and have the .lua file extension.