There is only one file type that uses the .MD5MESH extension:
id Tech 4 3D Mesh File
3D mesh file used by id Tech 4, a game engine used for Doom 3 and Quake 4; contains the polygonal mesh data for a model; used for storing skeletal meshes of characters, weapons, creatures, and other game objects; may be animated using a .MD5ANIM file and texturized with raster graphics stored in .TGA files.
MD5MESH files are part of the "MD5" model specification, which also includes MD5ANIM files for mesh animations and .MD5CAMERA files for camera paths during cut scenes. All of these files can be packaged into id Tech 4 packages, which are stored with the .PK4 extension.
MD5 meshes can be imported and exported using a variety of 3D modeling applications that have the appropriate plug-ins, including Maya, 3ds Max, Blender, Lightwave, and Softimage.
NOTE: While the MD5 format was developed by id Software, it is an open format that is publicly available for use. The id Tech 4 engine is also referred to as the "Doom 3 engine."
Updated: November 5, 2015