I assume you mean for assets? These are the supported file endings/types:
.prefab - Prefab files (json)
.ogg - Sounds and Music (or streaming and instant sounds)
.shape - Block Shape (specialised json mesh format)
.obj - Wavefront Mesh format
.png - Textures
.texinfo - Additional Texture info (json)
.glsl - Shaders
.info - Additional Shader info (json)
.mat - Material definition, combines shaders and their settings (json)
.block - Block definition (json)
.md5mesh - MD5 format skeletal mesh
.md5anim - MD5 format mesh animation
.fnt -
BMFont font definition
.atlas - Texture Atlas definition
And for NUI:
.skin - UI Skin (appearance)
.ui - UI Element definition