File extensions library

A file extension is only a label. Each page here explains what a format really is, how to check what is inside a file whatever its name, which free Windows programs open it, whether it can be converted and where the format falls short.

Video files

Containers that hold video, audio and subtitle tracks. Whether a file plays depends on the codecs inside, not only the extension.

.mkv — Matroska VideoMatroska container for video, many audio tracks, subtitles and chapters in one file. .webm — WebM videoOpen web video format: VP8, VP9 or AV1 video with Opus or Vorbis audio in a Matroska-based container.

Image files

Photo and graphics formats, what Windows opens out of the box and what needs an extension or another app.

.avif — AV1 Image File FormatImage format from the Alliance for Open Media that stores AV1-compressed pictures in a HEIF container.

Data and system files

Extensions that many different programs use for their own data. The name alone does not tell you what is inside.

.dat — Generic data fileGeneric extension for data files: winmail.dat email attachments, Video CD tracks, app and game data.