FLV MIME Type
video/x-flv identifies, how Flash Video files are structured, which applications open them, and how Poly supports FLV. MIME type details for video/x-flv
In active use| MIME type | video/x-flv |
|---|---|
| Extensions | .flv |
| Magic number | 46 4C 56 |
| First standardized | 2002 |
| Created by | Macromedia |
| Browser support | |
| Example applications | Adobe Animate, FFmpeg |
| Poly support | Yes. Poly recognizes `.flv` files and processes decodable audio and video for preview, search, metadata, and agent use. |
| Indexed by Poly | Yes. Poly indexes the filename, available technical metadata, spoken transcript, and sampled visual content. A transcript requires usable audio. |
| Preview in Poly | Yes. Poly provides a thumbnail, timeline previews, and in-app playback when the FLV's streams can be decoded. |
| Poly agent | Yes. The Poly agent can use supported visual content, transcript, and metadata. Results depend on decodable streams and audible speech. |
What does video/x-flv mean?
The video/x-flv media type identifies Flash Video files using the .flv extension. FLV is a container, so the label describes how timed audio, video, and data are packaged rather than naming one video codec. Adobe's specification permits one video stream and one audio stream in a file, synchronized by timestamps.1
The x- prefix is a clue that this is a conventional, unregistered media type. video/x-flv does not appear in IANA's current media type registry, although it remains the established identifier in format references and software.23 A server hosting an old FLV collection may still use it as the HTTP Content-Type.
.flv to .f4v or .mp4 does not convert the file.1How to identify an FLV file
An FLV file begins with the ASCII characters FLV, represented by these hexadecimal bytes:
46 4C 56
The next byte gives the file version, commonly 01. Header flags indicate whether audio and video tags are present, and a four-byte field gives the header length. The file body then alternates tags with back-pointers recording the size of the preceding tag.1
A tag can contain audio, video, or script data. Typical metadata includes duration, dimensions, frame rate, codecs, data rates, creation date, and file size, but the exact fields depend on the software that created the file.1
Do not rely on the .flv suffix alone. Renaming another video does not rewrite its container. Conversely, a valid FLV can be identified from its header even when its filename is missing the expected extension.
Which codecs can FLV contain?
The container and its codecs are separate compatibility layers. The Adobe specification assigns identifiers for several video encodings, including Sorenson H.263, Screen Video, On2 VP6, and AVC/H.264. Its audio identifiers include PCM, ADPCM, MP3, Nellymoser, AAC, and Speex.1
That variety explains why two FLV files may behave differently in the same player. Recognizing the FLV header is only the first step. The application must also support each contained stream, and older or unusual codecs are the most likely point of failure.
| Layer | What it tells you | Example |
|---|---|---|
| Media type | Expected container | video/x-flv |
| Extension | Filename convention | .flv |
| Container | Timing, tags, and stream packaging | FLV version 1 |
| Video codec | How picture frames are encoded | VP6 or H.264 |
| Audio codec | How sound samples are encoded | MP3 or AAC |
History and current browser support
Macromedia introduced Flash video capabilities in 2002 with Flash MX and Flash Player 6. FLV became a common delivery format for web video during the Flash era, and Adobe inherited it when it acquired Macromedia. The Library of Congress describes FLV as a fully documented binary delivery format developed by Macromedia and later owned by Adobe.34
FLV is now a legacy web format. Adobe ended Flash Player support on December 31, 2020 and blocked Flash content from running in Flash Player beginning January 12, 2021.5 FLV is not among the containers in MDN's current browser-support guide for HTML media.6 A desktop media application or conversion tool is the practical way to open an old file.
Useful options include:
- FFmpeg, whose official format documentation includes an FLV demuxer for reading FLV files.7
- Adobe Animate, which can import and work with FLV video in Flash authoring projects.8
- A maintained desktop media player that supports both the FLV container and the codecs inside your particular file.
FLV support in Poly
Poly recognizes .flv files as video/x-flv. When the streams can be decoded, Poly provides the same useful experience as other supported videos:
- A representative thumbnail and timeline previews for navigation.
- In-app playback that can jump to a relevant moment.
- Available technical details such as duration, dimensions, frame rate, codecs, color information, and audio tracks.
- Search by filename, spoken transcript, and sampled visual content.
- Access to supported visual content, transcript, and metadata through the Poly agent.
Transcripts require a usable audio track with audible speech. Silent video can still be searched visually. A damaged FLV or one using an unsupported legacy codec may have limited preview, indexing, or agent access.
See Supported File Types for the compact video feature comparison.
Converting FLV to a modern format
For everyday playback or web delivery, convert FLV to MP4 or WebM. FFmpeg can read FLV and is a common choice for inspecting, remuxing, or transcoding its streams.7
| Goal | Practical destination | Important check |
|---|---|---|
| Broad browser and device playback | MP4 with H.264 video and AAC audio | Verify that video, audio, duration, and synchronization survived |
| Open web delivery | WebM with VP9 or AV1 video and Opus audio | Test the browsers and devices your audience uses |
| Fast, lossless container change | Remux compatible streams without re-encoding | The destination must accept the existing codecs |
| Preservation master | Keep the original plus a documented modern derivative | Preserve original metadata and record conversion settings |
Re-encoding a lossy source can discard more detail. Keep the original FLV until the converted file has been checked from beginning to end. If the file contains uncommon metadata or data tags, ordinary video conversion may not preserve them.
Is FLV the same as SWF?
No. An SWF is a compiled Flash presentation or application that can contain a timeline, graphics, text, audio, video, and ActionScript. FLV is a video container that an SWF could load or embed.
| Format | Typical extension | Main purpose |
|---|---|---|
| FLV | .flv | Store synchronized video, audio, and timed data |
| SWF | .swf | Deliver a Flash presentation or interactive application |
| FLA | .fla | Store an editable Flash or Animate authoring project |
| F4V | .f4v | Store Flash-era media in an ISO Base Media File Format container |
This distinction matters for preservation. Converting an FLV normally preserves linear audiovisual content. Converting an interactive SWF to video records only one playback path and loses its application behavior.
Footnotes
- Adobe Systems. Flash Video File Format Specification, Version 10.1. The preserved Adobe specification defines FLV headers, tags, codecs, metadata, and encryption. ↩ ↩2 ↩3 ↩4 ↩5 ↩6
- Internet Assigned Numbers Authority. Media Types Registry. The current registry contains no
video/x-flventry. ↩ - Library of Congress. Macromedia Flash FLV Video File Format. Format description, identifiers, disclosure, and preservation context. ↩ ↩2
- The National Archives. PRONOM: Macromedia FLV, Version 1. Format history and identifier record. ↩
- Adobe. Adobe Flash Player End of Life. ↩ ↩2
- MDN Web Docs. Media Container Formats. ↩
- FFmpeg Project. FFmpeg Formats Documentation: FLV Demuxer. ↩ ↩2
- Adobe. Add or Incorporate Video in Adobe Animate. ↩