Reference
Supported file types
Atlas reads and previews images, video, audio, 3D models, and fonts out of the box, each with its own viewer, and plugins can teach it more.
More through plugins
A plugin can add new file types, giving Atlas a preview and details for them, so files it could not read on its own still show up in the grid like anything else. The page in the API Reference covers how.
Animated GIFs are treated as images: they get a GIF mark in the grid and open in the image viewer rather than the player.