There’s no official extension in the spec. OpenGL doesn’t handle loading shaders from files; you just pass in the shader code as a string, so there’s no specific file format.
However, glslang, Khronos’ reference GLSL compiler/validator, uses the following extensions to determine what type of shader that the file is for:
.vert– a vertex shader.tesc– a tessellation control shader.tese– a tessellation evaluation shader.geom– a geometry shader.frag– a fragment shader.comp– a compute shader