AVI MIME Type

What does video/x-msvideo mean?

The video/x-msvideo media type identifies AVI, Microsoft's Audio Video Interleave container. AVI stores one or more timed streams, usually video and audio, in a RIFF structure. The media type and the .avi extension identify the container, not the compression method inside it.¹²

That distinction explains why two .avi files can behave very differently. One might contain uncompressed frames and PCM audio, while another uses older compressed codecs that are no longer installed. A player has to understand the AVI container and every codec needed by its streams.

How to identify an AVI file

AVI is based on RIFF. A conventional file begins with the ASCII bytes RIFF, followed by a four-byte size field, then the form type AVI , including the trailing space. In hexadecimal, that pattern is:

52 49 46 46 ?? ?? ?? ?? 41 56 49 20

Checking only 52 49 46 46 is not enough. WAV, WebP, and other formats also use RIFF. A detector should confirm the AVI form type at bytes 8 through 11 and validate the following chunk structure.²⁴

Microsoft's AVI reference describes three important areas:²

• An hdrl list containing the main header and a description of each stream.
• A movi list containing the interleaved video frames and audio samples.
• An optional idx1 index that helps a player seek to media data.

FOURCC values identify chunks, stream types, and many video codecs. The main header can record details such as stream count, width, and height. Each stream then has its own header and format information.²

Why an AVI file may not play

AVI is a container rather than a single codec. Recognizing the container does not guarantee that a device can decode its streams. Microsoft notes that Windows Media Player can play AVI content only when the corresponding codecs are available, and lists examples ranging from DV and MJPEG to DivX, Cinepak, Indeo, and uncompressed video.⁵

This also makes browser support unreliable. Current web guidance recommends MP4 or WebM for general-purpose delivery and does not list AVI among broadly supported HTML video containers.⁶ An AVI that opens in a desktop player may still fail in a browser, or it may play video without audio when only one codec is supported.

VLC is a useful first choice for local playback because its published feature list includes AVI among its supported input containers.⁷ Windows also has native AVI container support through Media Foundation, but actual playback still depends on the codecs carried by the file.⁵⁸

AVI support in Poly

Poly recognizes .avi files as video/x-msvideo. When the contained streams can be decoded, Poly gives AVI the same practical video experience as other supported videos:

• A representative thumbnail and timeline previews for quick navigation.
• In-app playback that can jump to a matching moment.
• Technical metadata such as duration, dimensions, frame rate, codecs, color information, and audio-track details when available.
• Search by filename, spoken transcript, and sampled visual content.
• Access to supported visual content, transcript, and metadata through the Poly agent.

Speech search and transcript-based agent answers require an audio track with audible speech. Silent AVI files can still benefit from visual search. Damaged files or files containing unsupported legacy codecs may have limited preview, indexing, or agent access.

See Supported File Types for the compact feature comparison.

Converting AVI to MP4 or WebM

For web publishing, MP4 is usually the most compatible destination, while WebM is an open, web-focused alternative.⁶ Choose conversion settings based on what is inside the source:

Keep the original until you verify duration, audio synchronization, frame rate, orientation, and every required audio or subtitle stream. Old AVI files sometimes depend on uncommon codecs, so a conversion that silently drops a stream is not a successful preservation copy.

AVI, OpenDML AVI, and modern containers

The original AVI design dates to 1992. OpenDML extensions later expanded the format and addressed limitations of the original layout; the Library of Congress notes that these extensions are often informally called AVI 2.0.⁴ OpenDML AVI files still normally use .avi and the same container-level media type, so the filename alone does not reveal which indexing layout is present.

AVI remains useful for legacy collections, capture systems, and workflows built around particular codecs. For new delivery files, MP4 and WebM have a clearer web compatibility story. The important choice is not just “AVI versus MP4.” It is the combination of container, video codec, audio codec, metadata, and the software expected to read them.