There is only one file type that uses the .VMT extension:
Valve Material File
Material that can be applied to a two-dimensional surface of an object created with Valve's Source engine; contains information that the Source engine uses to visually, physically, and aurally simulate the surface.
Each VMT file includes the texture name, surface type, and shading parameters. It may also includes fallback information (for surfaces that cannot be rendered), and proxies (which change the material's rendering properties.
NOTE: VMT stands for "Valve Material Type."
Updated: August 24, 2014