Images (PNG, JPG, GIF) up to 8MB. Images will be downscaled to 1.6 megapixels (enough for a 1280x1280 image). Up to 4 images can be attached.
Animated GIFs are converted to soundless MP4s like on Imgur/Gfycat (GIFV). You can also upload soundless MP4 and WebM, which will be handled the same way.
More info here: https://docs.joinmastodon.org/user/posting/